Merge pull request 'refactor: update e2e page objects to use SmartLocator .loc() API' (#20) from feature/e2e-smartlocator-update into dev

Reviewed-on: #20

.woodpecker/pipeline.yml

@@ -21,21 +21,6 @@ steps:
       UV_PYTHON: "3.13"
     commands:
       - pip install uv
-      - cd ..
-      - |
-        cat > pyproject.toml << 'EOF'
-        [project]
-        name = "pyaqa"
-        version = "0.1.0"
-        requires-python = ">=3.13"
-        dependencies = []
-
-        [tool.uv.workspace]
-        members = ["blog.pyaqa.ru", "pytfm"]
-        EOF
-      - git clone https://git.pyaqa.ru/pi3c/pytfm.git
-      - cd $CI_WORKSPACE
-      - rm -rf .venv
       - uv sync --group lints --group tests --group types --group dev
 
   - name: lint
@@ -130,3 +115,33 @@ steps:
       - uv run --no-sync coverage combine .coverage.unit .coverage.integration
       - uv run --no-sync coverage report --fail-under=70 --include=app/*
       - uv run --no-sync coverage html
+
+  - name: pr-comment
+    image: python:3.13
+    volumes:
+      - /tmp/uv-cache:/root/.cache/uv
+    environment:
+      UV_CACHE_DIR: /root/.cache/uv
+      UV_LINK_MODE: copy
+      UV_PYTHON: "3.13"
+      GITEA_API_TOKEN:
+        from_secret: gitea_api_token
+    depends_on: [coverage, lint, type]
+    when:
+      event: [pull_request]
+    commands:
+      - pip install uv
+      - |
+        SHA7=$(printf '%.7s' "${CI_COMMIT_SHA:-unknown}")
+        COMMIT_URL="${CI_FORGE_URL}/${CI_REPO_OWNER}/${CI_REPO_NAME}/commit/${CI_COMMIT_SHA}"
+        SOURCE="${CI_COMMIT_SOURCE:-${CI_COMMIT_SOURCE_BRANCH:-?}}"
+        TARGET="${CI_COMMIT_TARGET:-${CI_COMMIT_TARGET_BRANCH:-?}}"
+        PIPELINE_URL="${CI_PIPELINE_URL:-}"
+        COVER=$(uv run --no-sync coverage report --include='app/*' | tail -1 | awk '{print $NF}')
+        if [ -z "$GITEA_API_TOKEN" ]; then
+          echo "pr-comment: GITEA_API_TOKEN not set, skipping"
+          exit 0
+        fi
+        FMT='{"body": "## CI Summary\n\n**Commit:** [`%s`](%s)\n**Branch:** `%s` → `%s`\n**Pipeline:** [View](%s)\n\n### Checks\n\n| Check | Status |\n|-------|--------|\n| Lint (ruff + isort) | ✅ |\n| Type check (mypy) | ✅ |\n| Unit tests | ✅ |\n| Integration tests | ✅ |\n| E2E tests | ✅ |\n| Coverage | **%s** |\n\n---\n*Reported by Woodpecker CI*"}'
+        BODY=$(printf "$FMT" "$SHA7" "$COMMIT_URL" "$SOURCE" "$TARGET" "$PIPELINE_URL" "$COVER")
+        curl -s -X POST "${CI_FORGE_URL}/api/v1/repos/${CI_REPO_OWNER}/${CI_REPO_NAME}/issues/${CI_COMMIT_PULL_REQUEST}/comments" -H "Authorization: token $${GITEA_API_TOKEN}" -H "Content-Type: application/json" --data-binary "$BODY"

AGENTS.md

@@ -406,6 +406,13 @@ User Acceptance
   v
 Commit (во все затронутые проекты)
   |-- blog, pytfm, pyaqa (root)
+  |
+  v
+Merge & Cleanup
+  |-- Дождаться влития PR в целевую ветку (dev/main)
+  |-- Переключиться на целевую ветку
+  |-- `git pull` — подтянуть изменения
+  |-- Удалить локальную фича-ветку: `git branch -d feature/{name}`
 ```
 
 ### Bugfix Lifecycle
@@ -441,6 +448,13 @@ User Acceptance
   |
   v
 Commit (во все затронутые проекты)
+  |
+  v
+Merge & Cleanup
+  |-- Дождаться влития PR в целевую ветку (dev/main)
+  |-- Переключиться на целевую ветку
+  |-- `git pull` — подтянуть изменения
+  |-- Удалить локальную фича-ветку: `git branch -d feature/{name}`
 ```
 
 ### Refactoring Lifecycle
@@ -481,6 +495,13 @@ User Acceptance (опционально)
   |
   v
 Commit (во все затронутые проекты)
+  |
+  v
+Merge & Cleanup
+  |-- Дождаться влития PR в целевую ветку (dev/main)
+  |-- Переключиться на целевую ветку
+  |-- `git pull` — подтянуть изменения
+  |-- Удалить локальную фича-ветку: `git branch -d feature/{name}`
 ```
 
 ### Branch Naming

app/application/__init__.py

@@ -4,14 +4,25 @@ This module re-exports all application layer components including
 DTOs, interfaces, and use cases for convenient importing.
 """
 
-from app.application.dtos import CreatePostDTO, PostResponseDTO, UpdatePostDTO
+from app.application.dtos import (
+    CommentResponseDTO,
+    CreateCommentDTO,
+    CreatePostDTO,
+    PostResponseDTO,
+    UpdatePostDTO,
+)
 from app.application.interfaces import TransactionManager
 from app.application.use_cases import (
+    CreateCommentUseCase,
     CreatePostUseCase,
+    DeleteCommentUseCase,
     DeletePostUseCase,
     GetPostUseCase,
+    ListCommentsUseCase,
     ListPostsUseCase,
     PublishPostUseCase,
+    ToggleCommentLikeUseCase,
+    TogglePostLikeUseCase,
     UpdatePostUseCase,
 )
 
@@ -19,6 +30,8 @@ __all__ = [
     "CreatePostDTO",
     "UpdatePostDTO",
     "PostResponseDTO",
+    "CreateCommentDTO",
+    "CommentResponseDTO",
     "TransactionManager",
     "CreatePostUseCase",
     "GetPostUseCase",
@@ -26,4 +39,9 @@ __all__ = [
     "DeletePostUseCase",
     "ListPostsUseCase",
     "PublishPostUseCase",
+    "TogglePostLikeUseCase",
+    "CreateCommentUseCase",
+    "DeleteCommentUseCase",
+    "ListCommentsUseCase",
+    "ToggleCommentLikeUseCase",
 ]

app/application/dtos/__init__.py

@@ -4,6 +4,13 @@ This module re-exports all Data Transfer Objects used in the
 application layer for data communication.
 """
 
+from app.application.dtos.comment import CommentResponseDTO, CreateCommentDTO
 from app.application.dtos.post import CreatePostDTO, PostResponseDTO, UpdatePostDTO
 
-__all__ = ["CreatePostDTO", "UpdatePostDTO", "PostResponseDTO"]
+__all__ = [
+    "CreatePostDTO",
+    "UpdatePostDTO",
+    "PostResponseDTO",
+    "CreateCommentDTO",
+    "CommentResponseDTO",
+]

app/application/dtos/comment.py

@@ -0,0 +1,55 @@
+"""DTOs for comment use cases.
+
+This module defines Data Transfer Objects used for communication between
+application layer comment use cases and presentation layer.
+"""
+
+from dataclasses import dataclass
+from datetime import datetime
+from uuid import UUID
+
+
+@dataclass(frozen=True)
+class CreateCommentDTO:
+    """DTO for creating a comment.
+
+    Carries comment creation data from API to use case.
+
+    Attributes:
+        post_id: UUID of the post to comment on.
+        author_id: Identifier of the comment author.
+        content: Comment content string (Markdown supported).
+        parent_id: Optional UUID of parent comment for replies.
+    """
+
+    post_id: UUID
+    author_id: str
+    content: str
+    parent_id: UUID | None = None
+
+
+@dataclass(frozen=True)
+class CommentResponseDTO:
+    """DTO for comment response.
+
+    Carries complete comment data for API responses.
+
+    Attributes:
+        id: Unique comment identifier.
+        post_id: UUID of the parent post.
+        author_id: Comment author identifier.
+        content: Comment content string.
+        parent_id: Optional UUID of parent comment.
+        like_count: Number of likes on this comment.
+        created_at: Creation timestamp.
+        updated_at: Last update timestamp.
+    """
+
+    id: UUID
+    post_id: UUID
+    author_id: str
+    content: str
+    parent_id: UUID | None = None
+    like_count: int = 0
+    created_at: datetime | None = None
+    updated_at: datetime | None = None

app/application/dtos/post.py

@@ -100,3 +100,5 @@ class PostResponseDTO:
     tags: list[str]
     created_at: datetime
     updated_at: datetime
+    like_count: int = 0
+    comment_count: int = 0

app/application/use_cases/__init__.py

@@ -4,11 +4,16 @@ This module re-exports all application use cases that implement
 business logic operations for the blog API.
 """
 
+from app.application.use_cases.create_comment import CreateCommentUseCase
 from app.application.use_cases.create_post import CreatePostUseCase
+from app.application.use_cases.delete_comment import DeleteCommentUseCase
 from app.application.use_cases.delete_post import DeletePostUseCase
 from app.application.use_cases.get_post import GetPostUseCase
+from app.application.use_cases.list_comments import ListCommentsUseCase
 from app.application.use_cases.list_posts import ListPostsUseCase
 from app.application.use_cases.publish_post import PublishPostUseCase
+from app.application.use_cases.toggle_comment_like import ToggleCommentLikeUseCase
+from app.application.use_cases.toggle_like import TogglePostLikeUseCase
 from app.application.use_cases.update_post import UpdatePostUseCase
 
 __all__ = [
@@ -18,4 +23,9 @@ __all__ = [
     "DeletePostUseCase",
     "ListPostsUseCase",
     "PublishPostUseCase",
+    "TogglePostLikeUseCase",
+    "CreateCommentUseCase",
+    "DeleteCommentUseCase",
+    "ListCommentsUseCase",
+    "ToggleCommentLikeUseCase",
 ]

app/application/use_cases/create_comment.py

@@ -0,0 +1,100 @@
+"""Create comment use case.
+
+This module implements the use case for creating comments on blog posts.
+Supports both top-level comments and nested replies via parent_id.
+"""
+
+from uuid import UUID
+
+from app.application.dtos.comment import CommentResponseDTO
+from app.application.interfaces import TransactionManager
+from app.domain.entities.comment import Comment
+from app.domain.exceptions import NotFoundException
+from app.domain.repositories import CommentRepository, PostRepository
+
+
+class CreateCommentUseCase:
+    """Use case for creating a comment on a blog post.
+
+    Handles top-level comments and replies to existing comments.
+    Validates that the target post exists before creating.
+
+    Attributes:
+        _post_repo: Repository for post data access.
+        _comment_repo: Repository for comment data access.
+        _tx_manager: Transaction manager for commit control.
+    """
+
+    def __init__(
+        self,
+        post_repo: PostRepository,
+        comment_repo: CommentRepository,
+        tx_manager: TransactionManager,
+    ) -> None:
+        """Initialize use case with dependencies.
+
+        Args:
+            post_repo: Repository for post operations.
+            comment_repo: Repository for comment operations.
+            tx_manager: Transaction manager instance.
+        """
+        self._post_repo = post_repo
+        self._comment_repo = comment_repo
+        self._tx_manager = tx_manager
+
+    async def execute(
+        self,
+        post_id: UUID,
+        author_id: str,
+        content: str,
+        parent_id: UUID | None = None,
+    ) -> CommentResponseDTO:
+        """Execute the use case to create a comment.
+
+        Args:
+            post_id: UUID of the post to comment on.
+            author_id: Identifier of the comment author.
+            content: Comment content (Markdown supported).
+            parent_id: Optional UUID of parent comment for replies.
+
+        Returns:
+            CommentResponseDTO with created comment data.
+
+        Raises:
+            NotFoundException: If the target post does not exist.
+        """
+        post = await self._post_repo.get_by_id(post_id)
+        if not post:
+            raise NotFoundException(f"Post with id '{post_id}' not found")
+
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str=content,
+            parent_id=parent_id,
+        )
+
+        await self._comment_repo.add(comment)
+        await self._tx_manager.commit()
+
+        return self._map_to_dto(comment)
+
+    def _map_to_dto(self, comment: Comment) -> CommentResponseDTO:
+        """Map domain entity to response DTO.
+
+        Args:
+            comment: Domain Comment entity.
+
+        Returns:
+            CommentResponseDTO with all comment attributes.
+        """
+        return CommentResponseDTO(
+            id=comment.id,
+            post_id=comment.post_id,
+            author_id=comment.author_id,
+            content=comment.content.value,
+            parent_id=comment.parent_id,
+            like_count=comment.like_count,
+            created_at=comment.created_at,
+            updated_at=comment.updated_at,
+        )

app/application/use_cases/create_post.py

@@ -91,6 +91,7 @@ class CreatePostUseCase:
             slug=post.slug.value,
             author_id=post.author_id,
             published=post.published,
+            like_count=post.like_count,
             tags=post.tags.copy(),
             created_at=post.created_at,
             updated_at=post.updated_at,

app/application/use_cases/delete_comment.py

@@ -0,0 +1,60 @@
+"""Delete comment use case.
+
+This module implements the use case for deleting comments.
+Users can delete their own comments.
+"""
+
+from uuid import UUID
+
+from app.application.interfaces import TransactionManager
+from app.domain.exceptions import ForbiddenException, NotFoundException
+from app.domain.repositories import CommentRepository
+
+
+class DeleteCommentUseCase:
+    """Use case for deleting a comment.
+
+    Allows users to delete their own comments.
+
+    Attributes:
+        _comment_repo: Repository for comment data access.
+        _tx_manager: Transaction manager for commit control.
+    """
+
+    def __init__(
+        self,
+        comment_repo: CommentRepository,
+        tx_manager: TransactionManager,
+    ) -> None:
+        """Initialize use case with dependencies.
+
+        Args:
+            comment_repo: Repository for comment operations.
+            tx_manager: Transaction manager instance.
+        """
+        self._comment_repo = comment_repo
+        self._tx_manager = tx_manager
+
+    async def execute(
+        self,
+        comment_id: UUID,
+        user_id: str,
+    ) -> None:
+        """Delete a comment.
+
+        Args:
+            comment_id: UUID of the comment to delete.
+            user_id: Identifier of the user requesting deletion.
+
+        Raises:
+            NotFoundException: If the comment does not exist.
+        """
+        comment = await self._comment_repo.get_by_id(comment_id)
+        if not comment:
+            raise NotFoundException(f"Comment with id '{comment_id}' not found")
+
+        if comment.author_id != user_id:
+            raise ForbiddenException("You are not allowed to delete this comment")
+
+        await self._comment_repo.delete(comment_id)
+        await self._tx_manager.commit()

app/application/use_cases/get_post.py

@@ -93,6 +93,7 @@ class GetPostUseCase:
             slug=post.slug.value,
             author_id=post.author_id,
             published=post.published,
+            like_count=post.like_count,
             tags=post.tags.copy(),
             created_at=post.created_at,
             updated_at=post.updated_at,

app/application/use_cases/list_comments.py

@@ -0,0 +1,63 @@
+"""List comments use case.
+
+This module implements the use case for listing comments on a blog post.
+"""
+
+from uuid import UUID
+
+from app.application.dtos.comment import CommentResponseDTO
+from app.domain.entities.comment import Comment
+from app.domain.repositories import CommentRepository
+
+
+class ListCommentsUseCase:
+    """Use case for listing comments on a blog post.
+
+    Retrieves all comments for a given post ordered by creation time.
+
+    Attributes:
+        _comment_repo: Repository for comment data access.
+    """
+
+    def __init__(
+        self,
+        comment_repo: CommentRepository,
+    ) -> None:
+        """Initialize use case with dependencies.
+
+        Args:
+            comment_repo: Repository for comment operations.
+        """
+        self._comment_repo = comment_repo
+
+    async def execute(self, post_id: UUID) -> list[CommentResponseDTO]:
+        """List all comments for a post.
+
+        Args:
+            post_id: UUID of the post.
+
+        Returns:
+            List of CommentResponseDTO for the post.
+        """
+        comments = await self._comment_repo.get_by_post(post_id)
+        return [self._map_to_dto(c) for c in comments]
+
+    def _map_to_dto(self, comment: Comment) -> CommentResponseDTO:
+        """Map domain entity to response DTO.
+
+        Args:
+            comment: Domain Comment entity.
+
+        Returns:
+            CommentResponseDTO with all comment attributes.
+        """
+        return CommentResponseDTO(
+            id=comment.id,
+            post_id=comment.post_id,
+            author_id=comment.author_id,
+            content=comment.content.value,
+            parent_id=comment.parent_id,
+            like_count=comment.like_count,
+            created_at=comment.created_at,
+            updated_at=comment.updated_at,
+        )

app/application/use_cases/list_posts.py

@@ -138,6 +138,7 @@ class ListPostsUseCase:
             slug=post.slug.value,
             author_id=post.author_id,
             published=post.published,
+            like_count=post.like_count,
             tags=post.tags.copy(),
             created_at=post.created_at,
             updated_at=post.updated_at,

app/application/use_cases/publish_post.py

@@ -125,6 +125,7 @@ class PublishPostUseCase:
             slug=post.slug.value,
             author_id=post.author_id,
             published=post.published,
+            like_count=post.like_count,
             tags=post.tags.copy(),
             created_at=post.created_at,
             updated_at=post.updated_at,

app/application/use_cases/toggle_comment_like.py

@@ -0,0 +1,96 @@
+"""Toggle comment like use case.
+
+This module implements the use case for toggling likes on comments.
+If the user already liked the comment, the like is removed (unlike).
+If not, a new like is added.
+"""
+
+from uuid import UUID
+
+from app.application.dtos.comment import CommentResponseDTO
+from app.application.interfaces import TransactionManager
+from app.domain.entities.comment import Comment
+from app.domain.entities.comment_like import CommentLike
+from app.domain.exceptions import NotFoundException
+from app.domain.repositories import CommentRepository
+
+
+class ToggleCommentLikeUseCase:
+    """Use case for toggling a like on a comment.
+
+    Handles like/unlike toggle logic. If the user has already liked
+    the comment, the like is removed. Otherwise, a new like is created.
+
+    Attributes:
+        _comment_repo: Repository for comment and like data access.
+        _tx_manager: Transaction manager for commit control.
+    """
+
+    def __init__(
+        self,
+        comment_repo: CommentRepository,
+        tx_manager: TransactionManager,
+    ) -> None:
+        """Initialize use case with dependencies.
+
+        Args:
+            comment_repo: Repository for comment and like operations.
+            tx_manager: Transaction manager instance.
+        """
+        self._comment_repo = comment_repo
+        self._tx_manager = tx_manager
+
+    async def execute(self, comment_id: UUID, liked_by: str) -> CommentResponseDTO:
+        """Toggle like on a comment.
+
+        If the user already liked the comment, remove the like.
+        Otherwise, add a new like.
+
+        Args:
+            comment_id: UUID of the comment to toggle like on.
+            liked_by: User ID.
+
+        Returns:
+            CommentResponseDTO with updated like_count.
+
+        Raises:
+            NotFoundException: If comment with given ID does not exist.
+        """
+        comment = await self._comment_repo.get_by_id(comment_id)
+        if not comment:
+            raise NotFoundException(f"Comment with id '{comment_id}' not found")
+
+        existing_like = await self._comment_repo.get_like(comment_id, liked_by)
+
+        if existing_like:
+            await self._comment_repo.remove_like(comment_id, liked_by)
+            comment.like_count = max(0, comment.like_count - 1)
+        else:
+            new_like = CommentLike(comment_id=comment_id, liked_by=liked_by)
+            await self._comment_repo.add_like(new_like)
+            comment.like_count += 1
+
+        await self._comment_repo.update(comment)
+        await self._tx_manager.commit()
+
+        return self._map_to_dto(comment)
+
+    def _map_to_dto(self, comment: Comment) -> CommentResponseDTO:
+        """Map domain entity to response DTO.
+
+        Args:
+            comment: Domain Comment entity.
+
+        Returns:
+            CommentResponseDTO with all comment attributes including like_count.
+        """
+        return CommentResponseDTO(
+            id=comment.id,
+            post_id=comment.post_id,
+            author_id=comment.author_id,
+            content=comment.content.value,
+            parent_id=comment.parent_id,
+            like_count=comment.like_count,
+            created_at=comment.created_at,
+            updated_at=comment.updated_at,
+        )

app/application/use_cases/toggle_like.py

@@ -0,0 +1,102 @@
+"""Toggle post like use case.
+
+This module implements the use case for toggling likes on blog posts.
+If the user already liked the post, the like is removed (unlike).
+If not, a new like is added.
+"""
+
+from uuid import UUID
+
+from app.application.dtos.post import PostResponseDTO
+from app.application.interfaces import TransactionManager
+from app.domain.entities import Post
+from app.domain.entities.like import PostLike
+from app.domain.exceptions import NotFoundException
+from app.domain.repositories import PostRepository
+
+
+class TogglePostLikeUseCase:
+    """Use case for toggling a like on a blog post.
+
+    Handles like/unlike toggle logic. If the user or device has already
+    liked the post, the like is removed. Otherwise, a new like is created.
+
+    Attributes:
+        _post_repo: Repository for post and like data access.
+        _tx_manager: Transaction manager for commit control.
+
+    Example:
+        >>> use_case = TogglePostLikeUseCase(post_repo, tx_manager)
+        >>> result = await use_case.execute("my-post-slug", "user-123")
+    """
+
+    def __init__(
+        self,
+        post_repo: PostRepository,
+        tx_manager: TransactionManager,
+    ) -> None:
+        """Initialize use case with dependencies.
+
+        Args:
+            post_repo: Repository for post and like operations.
+            tx_manager: Transaction manager instance.
+        """
+        self._post_repo = post_repo
+        self._tx_manager = tx_manager
+
+    async def execute(self, post_id: UUID, liked_by: str) -> PostResponseDTO:
+        """Toggle like on a post.
+
+        If the user/device already liked the post, remove the like.
+        Otherwise, add a new like.
+
+        Args:
+            post_id: UUID of the post to toggle like on.
+            liked_by: User ID or device identifier.
+
+        Returns:
+            PostResponseDTO with updated like_count.
+
+        Raises:
+            NotFoundException: If post with given ID does not exist.
+        """
+        post = await self._post_repo.get_by_id(post_id)
+        if not post:
+            raise NotFoundException(f"Post with id '{post_id}' not found")
+
+        existing_like = await self._post_repo.get_like(post_id, liked_by)
+
+        if existing_like:
+            await self._post_repo.remove_like(post_id, liked_by)
+            post.like_count = max(0, post.like_count - 1)
+        else:
+            new_like = PostLike(post_id=post_id, liked_by=liked_by)
+            await self._post_repo.add_like(new_like)
+            post.like_count += 1
+
+        await self._post_repo.update(post)
+        await self._tx_manager.commit()
+
+        return self._map_to_dto(post)
+
+    def _map_to_dto(self, post: Post) -> PostResponseDTO:
+        """Map domain entity to response DTO.
+
+        Args:
+            post: Domain post entity.
+
+        Returns:
+            PostResponseDTO with all post attributes including like_count.
+        """
+        return PostResponseDTO(
+            id=post.id,
+            title=post.title.value,
+            content=post.content.value,
+            slug=post.slug.value,
+            author_id=post.author_id,
+            published=post.published,
+            like_count=post.like_count,
+            tags=post.tags.copy(),
+            created_at=post.created_at,
+            updated_at=post.updated_at,
+        )

app/application/use_cases/update_post.py

@@ -108,6 +108,7 @@ class UpdatePostUseCase:
             slug=post.slug.value,
             author_id=post.author_id,
             published=post.published,
+            like_count=post.like_count,
             tags=post.tags.copy(),
             created_at=post.created_at,
             updated_at=post.updated_at,

app/domain/__init__.py

@@ -4,7 +4,7 @@ This module re-exports all domain layer components including
 entities, value objects, repositories, and exceptions.
 """
 
-from app.domain.entities import BaseEntity, Post
+from app.domain.entities import BaseEntity, Comment, CommentLike, Post, PostLike
 from app.domain.exceptions import (
     AlreadyExistsException,
     DomainException,
@@ -13,18 +13,22 @@ from app.domain.exceptions import (
     UnauthorizedException,
     ValidationException,
 )
-from app.domain.repositories import PostRepository, Repository
+from app.domain.repositories import CommentRepository, PostRepository, Repository
 from app.domain.value_objects import Content, Slug, Title, ValueObject
 
 __all__ = [
     "BaseEntity",
     "Post",
+    "PostLike",
+    "Comment",
+    "CommentLike",
     "ValueObject",
     "Title",
     "Content",
     "Slug",
     "Repository",
     "PostRepository",
+    "CommentRepository",
     "DomainException",
     "ValidationException",
     "NotFoundException",

app/domain/entities/__init__.py

@@ -5,6 +5,9 @@ core business objects with identity.
 """
 
 from app.domain.entities.base import BaseEntity
+from app.domain.entities.comment import Comment
+from app.domain.entities.comment_like import CommentLike
+from app.domain.entities.like import PostLike
 from app.domain.entities.post import Post
 
-__all__ = ["BaseEntity", "Post"]
+__all__ = ["BaseEntity", "Post", "PostLike", "Comment", "CommentLike"]

app/domain/entities/comment.py

@@ -0,0 +1,79 @@
+"""Domain entity for Comment.
+
+This module defines the Comment entity that represents a comment on a blog
+post. Comments can be top-level (parent_id=None) or replies to other
+comments (parent_id set).
+"""
+
+from dataclasses import dataclass
+from typing import Any
+from uuid import UUID
+
+from app.domain.entities.base import BaseEntity
+from app.domain.value_objects.comment_content import CommentContent
+
+
+@dataclass(kw_only=True)
+class Comment(BaseEntity):
+    """Comment domain entity.
+
+    Represents a comment on a blog post with optional parent reference
+    for nested replies. Supports Markdown content and like tracking.
+
+    Attributes:
+        post_id: UUID of the post this comment belongs to.
+        author_id: Identifier of the comment author.
+        content: CommentContent value object with Markdown text.
+        parent_id: UUID of parent comment for replies, or None.
+        like_count: Number of likes on this comment.
+    """
+
+    post_id: UUID
+    author_id: str
+    content: CommentContent
+    parent_id: UUID | None = None
+    like_count: int = 0
+
+    def to_dict(self) -> dict[str, Any]:
+        """Convert entity to dictionary.
+
+        Returns:
+            Dictionary representation with all comment attributes.
+        """
+        return {
+            "id": str(self.id),
+            "post_id": str(self.post_id),
+            "author_id": self.author_id,
+            "content": self.content.value,
+            "parent_id": str(self.parent_id) if self.parent_id else None,
+            "like_count": self.like_count,
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+        }
+
+    @classmethod
+    def create(
+        cls,
+        post_id: UUID,
+        author_id: str,
+        content_str: str,
+        parent_id: UUID | None = None,
+    ) -> "Comment":
+        """Factory method to create a new comment.
+
+        Args:
+            post_id: UUID of the post to comment on.
+            author_id: Identifier of the comment author.
+            content_str: Comment content string (Markdown supported).
+            parent_id: Optional UUID of parent comment for replies.
+
+        Returns:
+            New Comment instance with validated content.
+        """
+        content = CommentContent(content_str)
+        return cls(
+            post_id=post_id,
+            author_id=author_id,
+            content=content,
+            parent_id=parent_id,
+        )

app/domain/entities/comment_like.py

@@ -0,0 +1,40 @@
+"""Domain entity for CommentLike.
+
+This module defines the CommentLike entity that tracks which users
+have liked which comments.
+"""
+
+from dataclasses import dataclass
+from typing import Any
+from uuid import UUID
+
+from app.domain.entities.base import BaseEntity
+
+
+@dataclass(kw_only=True)
+class CommentLike(BaseEntity):
+    """Comment like domain entity.
+
+    Tracks a like on a comment by a user. Each like is uniquely
+    identified by its entity ID.
+
+    Attributes:
+        comment_id: UUID of the liked comment.
+        liked_by: Identifier of the user who liked.
+    """
+
+    comment_id: UUID
+    liked_by: str
+
+    def to_dict(self) -> dict[str, Any]:
+        """Convert entity to dictionary.
+
+        Returns:
+            Dictionary with all CommentLike attributes.
+        """
+        return {
+            "id": str(self.id),
+            "comment_id": str(self.comment_id),
+            "liked_by": self.liked_by,
+            "created_at": self.created_at.isoformat(),
+        }

app/domain/entities/like.py

@@ -0,0 +1,40 @@
+"""Domain entity for PostLike.
+
+This module defines the PostLike entity that tracks which users
+or devices have liked which posts.
+"""
+
+from dataclasses import dataclass
+from typing import Any
+from uuid import UUID
+
+from app.domain.entities.base import BaseEntity
+
+
+@dataclass(kw_only=True)
+class PostLike(BaseEntity):
+    """Post like domain entity.
+
+    Tracks a like on a blog post by a user or device.
+    Each like is uniquely identified by its entity ID.
+
+    Attributes:
+        post_id: UUID of the liked post.
+        liked_by: Identifier of the user or device that liked.
+    """
+
+    post_id: UUID
+    liked_by: str
+
+    def to_dict(self) -> dict[str, Any]:
+        """Convert entity to dictionary.
+
+        Returns:
+            Dictionary with all PostLike attributes.
+        """
+        return {
+            "id": str(self.id),
+            "post_id": str(self.post_id),
+            "liked_by": self.liked_by,
+            "created_at": self.created_at.isoformat(),
+        }

app/domain/entities/post.py

@@ -44,6 +44,7 @@ class Post(BaseEntity):
     slug: Slug
     author_id: str
     published: bool = False
+    like_count: int = 0
     tags: list[str] = field(default_factory=list)
 
     def publish(self) -> None:
@@ -114,6 +115,7 @@ class Post(BaseEntity):
             "slug": self.slug.value,
             "author_id": self.author_id,
             "published": self.published,
+            "like_count": self.like_count,
             "tags": self.tags.copy(),
             "created_at": self.created_at.isoformat(),
             "updated_at": self.updated_at.isoformat(),

app/domain/repositories/__init__.py

@@ -5,6 +5,7 @@ the contract for data access operations.
 """
 
 from app.domain.repositories.base import Repository
+from app.domain.repositories.comment import CommentRepository
 from app.domain.repositories.post import PostRepository
 
-__all__ = ["Repository", "PostRepository"]
+__all__ = ["Repository", "PostRepository", "CommentRepository"]

app/domain/repositories/comment.py

@@ -0,0 +1,80 @@
+"""Comment repository interface.
+
+This module defines the repository interface for Comment entities
+including nested comment queries and like management.
+"""
+
+from abc import abstractmethod
+from uuid import UUID
+
+from app.domain.entities.comment import Comment
+from app.domain.entities.comment_like import CommentLike
+from app.domain.repositories.base import Repository
+
+
+class CommentRepository(Repository[Comment]):
+    """Repository interface for Comments.
+
+    Extends the generic repository with comment-specific operations
+    including post-based listing and like management.
+
+    Example:
+        >>> comments = await repo.get_by_post(post_id)
+        >>> like = await repo.get_like(comment_id, "user-123")
+    """
+
+    @abstractmethod
+    async def get_by_post(self, post_id: UUID) -> list[Comment]:
+        """Get all comments for a post, ordered by creation time.
+
+        Args:
+            post_id: UUID of the post.
+
+        Returns:
+            List of Comment entities for the post.
+        """
+        ...
+
+    @abstractmethod
+    async def get_like(self, comment_id: UUID, liked_by: str) -> CommentLike | None:
+        """Get a like by comment and user.
+
+        Args:
+            comment_id: UUID of the comment.
+            liked_by: User ID.
+
+        Returns:
+            CommentLike if found, None otherwise.
+        """
+        ...
+
+    @abstractmethod
+    async def add_like(self, like: CommentLike) -> None:
+        """Add a new like to a comment.
+
+        Args:
+            like: CommentLike entity to add.
+        """
+        ...
+
+    @abstractmethod
+    async def count_by_post(self, post_id: UUID) -> int:
+        """Get comment count for a post.
+
+        Args:
+            post_id: UUID of the post.
+
+        Returns:
+            Number of comments on the post.
+        """
+        ...
+
+    @abstractmethod
+    async def remove_like(self, comment_id: UUID, liked_by: str) -> None:
+        """Remove a like from a comment by user.
+
+        Args:
+            comment_id: UUID of the comment.
+            liked_by: User ID.
+        """
+        ...

app/domain/repositories/post.py

@@ -1,11 +1,14 @@
 """Post repository interface.
 
 This module extends the base repository interface with post-specific
-query methods including slug lookup, author filtering, and search.
+query methods including slug lookup, author filtering, search, and
+like management.
 """
 
 from abc import abstractmethod
+from uuid import UUID
 
+from app.domain.entities.like import PostLike
 from app.domain.entities.post import Post
 from app.domain.repositories.base import Repository
 
@@ -101,6 +104,38 @@ class PostRepository(Repository[Post]):
         """
         ...
 
+    @abstractmethod
+    async def get_like(self, post_id: UUID, liked_by: str) -> PostLike | None:
+        """Get a like by post and user/device.
+
+        Args:
+            post_id: UUID of the post.
+            liked_by: User ID or device ID.
+
+        Returns:
+            PostLike if found, None otherwise.
+        """
+        ...
+
+    @abstractmethod
+    async def add_like(self, like: PostLike) -> None:
+        """Add a new like.
+
+        Args:
+            like: PostLike entity to add.
+        """
+        ...
+
+    @abstractmethod
+    async def remove_like(self, post_id: UUID, liked_by: str) -> None:
+        """Remove a like by post and user/device.
+
+        Args:
+            post_id: UUID of the post.
+            liked_by: User ID or device ID.
+        """
+        ...
+
     @abstractmethod
     async def search(
         self,

app/domain/value_objects/__init__.py

@@ -5,8 +5,9 @@ immutable validated domain concepts.
 """
 
 from app.domain.value_objects.base import ValueObject
+from app.domain.value_objects.comment_content import CommentContent
 from app.domain.value_objects.content import Content
 from app.domain.value_objects.slug import Slug
 from app.domain.value_objects.title import Title
 
-__all__ = ["ValueObject", "Title", "Content", "Slug"]
+__all__ = ["ValueObject", "Title", "Content", "Slug", "CommentContent"]

app/domain/value_objects/comment_content.py

@@ -0,0 +1,47 @@
+"""Value object for comment content.
+
+This module defines the CommentContent value object that validates
+and encapsulates comment text content.
+"""
+
+from dataclasses import dataclass
+
+from app.domain.value_objects.base import ValueObject
+
+
+@dataclass(frozen=True, slots=True)
+class CommentContent(ValueObject[str]):
+    """Comment content value object.
+
+    Wraps and validates comment content ensuring it meets length
+    requirements and is not empty.
+
+    Attributes:
+        value: The comment content string.
+        MAX_LENGTH: Maximum allowed content length (5000 characters).
+
+    Raises:
+        ValueError: If content is empty or too long.
+
+    Example:
+        >>> content = CommentContent("This is a **bold** comment.")
+        >>> content.value
+        'This is a **bold** comment.'
+    """
+
+    MAX_LENGTH: int = 5000
+
+    def _validate(self) -> None:
+        """Validate comment content.
+
+        Checks that content is a non-empty string within length bounds.
+
+        Raises:
+            ValueError: If content fails validation criteria.
+        """
+        if not isinstance(self.value, str):
+            raise ValueError("Comment content must be a string")
+        if not self.value.strip():
+            raise ValueError("Comment content cannot be empty")
+        if len(self.value) > self.MAX_LENGTH:
+            raise ValueError(f"Comment content must be at most {self.MAX_LENGTH} characters")

app/infrastructure/database/models.py

@@ -4,11 +4,13 @@ This module defines the database ORM models that map to database tables.
 Models are used by repositories for data persistence.
 """
 
+from __future__ import annotations
+
 from datetime import UTC, datetime
 from uuid import uuid4
 
-from sqlalchemy import JSON, Boolean, DateTime, String, Text
-from sqlalchemy.orm import Mapped, declarative_base, mapped_column
+from sqlalchemy import JSON, Boolean, DateTime, ForeignKey, Integer, String, Text
+from sqlalchemy.orm import Mapped, declarative_base, mapped_column, relationship
 
 Base = declarative_base()
 
@@ -42,6 +44,13 @@ class PostORM(Base):  # type: ignore[valid-type,misc]
     slug: Mapped[str] = mapped_column(String(200), nullable=False, unique=True, index=True)
     author_id: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
     published: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False, index=True)
+    like_count: Mapped[int] = mapped_column(default=0, nullable=False)
+    likes: Mapped[list[PostLikeORM]] = relationship(
+        back_populates="post", cascade="all, delete-orphan"
+    )
+    comments: Mapped[list[CommentORM]] = relationship(
+        back_populates="post", cascade="all, delete-orphan"
+    )
     tags: Mapped[list[str]] = mapped_column(JSON, default=list)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
@@ -54,3 +63,116 @@ class PostORM(Base):  # type: ignore[valid-type,misc]
         onupdate=lambda: datetime.now(UTC),
         nullable=False,
     )
+
+
+class PostLikeORM(Base):  # type: ignore[valid-type,misc]
+    """SQLAlchemy model for PostLike.
+
+    Database table representation of post likes.
+    Maps to the 'post_likes' table tracking which users/devices liked which posts.
+
+    Attributes:
+        id: Primary key as UUID string.
+        post_id: Foreign key to the liked post.
+        liked_by: User ID or device identifier.
+        created_at: Creation timestamp.
+    """
+
+    __tablename__ = "post_likes"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid4()))
+    post_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("posts.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    liked_by: Mapped[str] = mapped_column(String(200), nullable=False, index=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        nullable=False,
+    )
+
+    post: Mapped[PostORM] = relationship(back_populates="likes")
+
+
+class CommentORM(Base):  # type: ignore[valid-type,misc]
+    """SQLAlchemy model for Comment.
+
+    Database table representation of comments on blog posts.
+    Maps to the 'comments' table with support for nested replies.
+
+    Attributes:
+        id: Primary key as UUID string.
+        post_id: Foreign key to the parent post.
+        author_id: Comment author reference.
+        content: Comment text content (Markdown supported).
+        parent_id: Optional foreign key to parent comment (replies).
+        like_count: Number of likes on this comment.
+        created_at: Creation timestamp.
+        updated_at: Last update timestamp.
+    """
+
+    __tablename__ = "comments"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid4()))
+    post_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("posts.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    author_id: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    content: Mapped[str] = mapped_column(Text, nullable=False)
+    parent_id: Mapped[str | None] = mapped_column(
+        String(36),
+        ForeignKey("comments.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    like_count: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        nullable=False,
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        onupdate=lambda: datetime.now(UTC),
+        nullable=False,
+    )
+
+    post: Mapped[PostORM] = relationship(back_populates="comments")
+    replies: Mapped[list[CommentORM]] = relationship(
+        back_populates="parent",
+    )
+    parent: Mapped[CommentORM | None] = relationship(
+        back_populates="replies",
+        remote_side="CommentORM.id",
+    )
+
+
+class CommentLikeORM(Base):  # type: ignore[valid-type,misc]
+    """SQLAlchemy model for CommentLike.
+
+    Database table representation of comment likes.
+    Maps to the 'comment_likes' table tracking which users liked which comments.
+
+    Attributes:
+        id: Primary key as UUID string.
+        comment_id: Foreign key to the liked comment.
+        liked_by: User identifier.
+        created_at: Creation timestamp.
+    """
+
+    __tablename__ = "comment_likes"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid4()))
+    comment_id: Mapped[str] = mapped_column(
+        String(36),
+        ForeignKey("comments.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    liked_by: Mapped[str] = mapped_column(String(200), nullable=False, index=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        nullable=False,
+    )

app/infrastructure/di/providers.py

@@ -10,18 +10,24 @@ from dishka import Provider, Scope, provide
 from sqlalchemy.ext.asyncio import AsyncEngine, AsyncSession
 
 from app.application import (
+    CreateCommentUseCase,
     CreatePostUseCase,
+    DeleteCommentUseCase,
     DeletePostUseCase,
     GetPostUseCase,
+    ListCommentsUseCase,
     ListPostsUseCase,
     PublishPostUseCase,
+    ToggleCommentLikeUseCase,
+    TogglePostLikeUseCase,
     UpdatePostUseCase,
 )
 from app.application.interfaces import TransactionManager
-from app.domain.repositories import PostRepository
+from app.domain.repositories import CommentRepository, PostRepository
 from app.infrastructure.auth import KeycloakAuthClient, MockKeycloakClient
 from app.infrastructure.config.settings import settings
 from app.infrastructure.database.connection import AsyncSessionLocal, engine
+from app.infrastructure.repositories.comment import SQLAlchemyCommentRepository
 from app.infrastructure.repositories.post import SQLAlchemyPostRepository
 
 
@@ -80,6 +86,18 @@ class RepositoryProvider(Provider):
         """
         return SQLAlchemyPostRepository(session)
 
+    @provide(scope=Scope.REQUEST)
+    def get_comment_repository(self, session: AsyncSession) -> CommentRepository:
+        """Provide CommentRepository implementation.
+
+        Args:
+            session: Database session from DI container.
+
+        Returns:
+            SQLAlchemyCommentRepository instance.
+        """
+        return SQLAlchemyCommentRepository(session)
+
 
 class TransactionManagerProvider(Provider):
     """Provider for transaction manager.
@@ -236,6 +254,106 @@ class UseCaseProvider(Provider):
             tx_manager=tx_manager,
         )
 
+    @provide(scope=Scope.REQUEST)
+    def get_toggle_like_use_case(
+        self,
+        post_repo: PostRepository,
+        tx_manager: TransactionManager,
+    ) -> TogglePostLikeUseCase:
+        """Provide TogglePostLikeUseCase.
+
+        Args:
+            post_repo: Post repository dependency.
+            tx_manager: Transaction manager dependency.
+
+        Returns:
+            Configured TogglePostLikeUseCase instance.
+        """
+        return TogglePostLikeUseCase(
+            post_repo=post_repo,
+            tx_manager=tx_manager,
+        )
+
+    @provide(scope=Scope.REQUEST)
+    def get_create_comment_use_case(
+        self,
+        post_repo: PostRepository,
+        comment_repo: CommentRepository,
+        tx_manager: TransactionManager,
+    ) -> CreateCommentUseCase:
+        """Provide CreateCommentUseCase.
+
+        Args:
+            post_repo: Post repository dependency.
+            comment_repo: Comment repository dependency.
+            tx_manager: Transaction manager dependency.
+
+        Returns:
+            Configured CreateCommentUseCase instance.
+        """
+        return CreateCommentUseCase(
+            post_repo=post_repo,
+            comment_repo=comment_repo,
+            tx_manager=tx_manager,
+        )
+
+    @provide(scope=Scope.REQUEST)
+    def get_list_comments_use_case(
+        self,
+        comment_repo: CommentRepository,
+    ) -> ListCommentsUseCase:
+        """Provide ListCommentsUseCase.
+
+        Args:
+            comment_repo: Comment repository dependency.
+
+        Returns:
+            Configured ListCommentsUseCase instance.
+        """
+        return ListCommentsUseCase(
+            comment_repo=comment_repo,
+        )
+
+    @provide(scope=Scope.REQUEST)
+    def get_delete_comment_use_case(
+        self,
+        comment_repo: CommentRepository,
+        tx_manager: TransactionManager,
+    ) -> DeleteCommentUseCase:
+        """Provide DeleteCommentUseCase.
+
+        Args:
+            comment_repo: Comment repository dependency.
+            tx_manager: Transaction manager dependency.
+
+        Returns:
+            Configured DeleteCommentUseCase instance.
+        """
+        return DeleteCommentUseCase(
+            comment_repo=comment_repo,
+            tx_manager=tx_manager,
+        )
+
+    @provide(scope=Scope.REQUEST)
+    def get_toggle_comment_like_use_case(
+        self,
+        comment_repo: CommentRepository,
+        tx_manager: TransactionManager,
+    ) -> ToggleCommentLikeUseCase:
+        """Provide ToggleCommentLikeUseCase.
+
+        Args:
+            comment_repo: Comment repository dependency.
+            tx_manager: Transaction manager dependency.
+
+        Returns:
+            Configured ToggleCommentLikeUseCase instance.
+        """
+        return ToggleCommentLikeUseCase(
+            comment_repo=comment_repo,
+            tx_manager=tx_manager,
+        )
+
 
 class KeycloakProvider(Provider):
     """Provider for Keycloak authentication client.

app/infrastructure/i18n/__init__.py

@@ -0,0 +1,9 @@
+"""Internationalization support for the blog application.
+
+This package provides translation dictionaries and the translation service
+for localizing the web UI into multiple languages.
+"""
+
+from app.infrastructure.i18n.translator import SUPPORTED_LOCALES, TranslationService, _
+
+__all__ = ["SUPPORTED_LOCALES", "TranslationService", "_"]

app/infrastructure/i18n/translations.py

@@ -0,0 +1,377 @@
+"""Translation dictionaries for i18n support.
+
+This module provides translation dictionaries for all supported locales.
+Translations are organized by feature area for maintainability.
+Keys use dot-separated namespacing to avoid collisions.
+"""
+
+TRANSLATIONS: dict[str, dict[str, str]] = {
+    "en": {
+        "nav.home": "Home",
+        "nav.posts": "Posts",
+        "nav.about": "About",
+        "header.logo": "Blog",
+        "header.profile": "Profile",
+        "header.new_post": "New Post",
+        "header.sign_out": "Sign Out",
+        "header.sign_in": "Sign In",
+        "header.toggle_menu": "Toggle menu",
+        "header.toggle_theme": "Toggle dark mode",
+        "header.lang_switcher": "Language",
+        "footer.copyright": "© 2026 Blog. All rights reserved.",
+        "footer.about": "About",
+        "footer.privacy": "Privacy",
+        "footer.terms": "Terms",
+        "footer.api": "API",
+        "home.title": "Blog - Home",
+        "home.meta_description": "Discover stories, thinking, and expertise from writers on any topic. A modern blog built with FastAPI.",
+        "home.meta_keywords": "blog, articles, posts, writing, fastapi, python",
+        "home.page_title": "Latest Posts",
+        "home.page_subtitle": "Discover stories, thinking, and expertise from writers on any topic.",
+        "home.write_post": "Write a Post",
+        "home.status_published": "Published",
+        "home.status_draft": "Draft",
+        "home.read_more": "Read more",
+        "home.pagination_previous": "Previous",
+        "home.pagination_next": "Next",
+        "home.empty_title": "No posts yet",
+        "home.empty_description": "Be the first to write a post!",
+        "home.empty_action": "Create your first post",
+        "post.status_published": "Published",
+        "post.status_draft": "Draft",
+        "post.back_to_posts": "Back to posts",
+        "post.edit": "Edit",
+        "post.delete": "Delete",
+        "post.delete_confirm": "Are you sure you want to delete this post?",
+        "post.comments": "Comments",
+        "post.write_comment": "Write a Comment",
+        "post.comment_placeholder": "Write a comment... Markdown is supported.",
+        "post.replying_to": "Replying to",
+        "post.cancel_reply": "Cancel reply",
+        "post.cancel": "Cancel",
+        "post.submit_comment": "Submit Comment",
+        "post.reply": "Reply",
+        "post.no_comments": "No comments yet. Be the first to comment!",
+        "post.comment_error": "Failed to post comment. Please try again.",
+        "post_form.title_edit": "Edit Post",
+        "post_form.title_new": "New Post",
+        "post_form.page_title_edit": "Edit Post",
+        "post_form.page_title_new": "Create New Post",
+        "post_form.label_title": "Title",
+        "post_form.placeholder_title": "Enter post title",
+        "post_form.hint_title": "A catchy title for your post",
+        "post_form.label_content": "Content",
+        "post_form.placeholder_content": "Write your post content here...",
+        "post_form.hint_content": "The main content of your post. Markdown is supported.",
+        "post_form.label_tags": "Tags",
+        "post_form.placeholder_tags": "python, fastapi, tutorial",
+        "post_form.hint_tags": "Comma-separated list of tags",
+        "post_form.cancel": "Cancel",
+        "post_form.save_draft": "Save as Draft",
+        "post_form.update_post": "Update Post",
+        "post_form.publish_post": "Publish Post",
+        "profile.title": "User Profile",
+        "profile.email": "Email:",
+        "profile.not_provided": "Not provided",
+        "profile.user_id": "User ID:",
+        "profile.name": "Name:",
+        "profile.back_home": "Back to Home",
+        "profile.new_post": "New Post",
+        "about.title": "About",
+        "about.page_title": "About",
+        "about.description": "A modern blog built with FastAPI and Domain-Driven Design architecture.",
+        "about.signed_in": "Signed in as {username}.",
+        "about.browsing_guest": "You are browsing as a guest.",
+        "about.back_home": "Back to Home",
+        "flash.post_published": "Post published successfully!",
+        "flash.post_saved_draft": "Post saved as draft!",
+        "flash.post_updated": "Post updated successfully!",
+        "flash.post_deleted": "Post deleted successfully!",
+        "flash.post_not_found": "Post not found.",
+        "base.close_message": "Close message",
+        "base.default_title": "Blog",
+        "base.meta_description": "Blog - A modern blogging platform built with FastAPI",
+        "base.meta_keywords": "blog, articles, posts, writing",
+        "base.meta_author": "Blog Team",
+        "lang.en": "English",
+        "lang.ru": "Русский",
+        "lang.fr": "Français",
+        "lang.de": "Deutsch",
+    },
+    "ru": {
+        "nav.home": "Главная",
+        "nav.posts": "Статьи",
+        "nav.about": "О нас",
+        "header.logo": "Блог",
+        "header.profile": "Профиль",
+        "header.new_post": "Новая статья",
+        "header.sign_out": "Выйти",
+        "header.sign_in": "Войти",
+        "header.toggle_menu": "Открыть меню",
+        "header.toggle_theme": "Сменить тему",
+        "header.lang_switcher": "Язык",
+        "footer.copyright": "© 2026 Блог. Все права защищены.",
+        "footer.about": "О нас",
+        "footer.privacy": "Конфиденциальность",
+        "footer.terms": "Условия",
+        "footer.api": "API",
+        "home.title": "Блог — Главная",
+        "home.meta_description": "Откройте для себя истории, мысли и опыт авторов на любую тему. Современный блог на FastAPI.",
+        "home.meta_keywords": "блог, статьи, посты, fastapi, python",
+        "home.page_title": "Последние статьи",
+        "home.page_subtitle": "Откройте для себя истории, мысли и опыт авторов на любую тему.",
+        "home.write_post": "Написать статью",
+        "home.status_published": "Опубликовано",
+        "home.status_draft": "Черновик",
+        "home.read_more": "Читать далее",
+        "home.pagination_previous": "Назад",
+        "home.pagination_next": "Вперёд",
+        "home.empty_title": "Статей пока нет",
+        "home.empty_description": "Будьте первым, кто напишет статью!",
+        "home.empty_action": "Создать первую статью",
+        "post.status_published": "Опубликовано",
+        "post.status_draft": "Черновик",
+        "post.back_to_posts": "К списку статей",
+        "post.edit": "Редактировать",
+        "post.delete": "Удалить",
+        "post.delete_confirm": "Вы уверены, что хотите удалить эту статью?",
+        "post.comments": "Комментарии",
+        "post.write_comment": "Написать комментарий",
+        "post.comment_placeholder": "Напишите комментарий... Поддерживается Markdown.",
+        "post.replying_to": "Ответ",
+        "post.cancel_reply": "Отменить ответ",
+        "post.cancel": "Отмена",
+        "post.submit_comment": "Отправить",
+        "post.reply": "Ответить",
+        "post.no_comments": "Пока нет комментариев. Будьте первым!",
+        "post.comment_error": "Не удалось отправить комментарий. Попробуйте снова.",
+        "post_form.title_edit": "Редактировать статью",
+        "post_form.title_new": "Новая статья",
+        "post_form.page_title_edit": "Редактировать статью",
+        "post_form.page_title_new": "Создать новую статью",
+        "post_form.label_title": "Заголовок",
+        "post_form.placeholder_title": "Введите заголовок статьи",
+        "post_form.hint_title": "Запоминающийся заголовок для вашей статьи",
+        "post_form.label_content": "Содержание",
+        "post_form.placeholder_content": "Напишите содержание статьи здесь...",
+        "post_form.hint_content": "Основное содержание вашей статьи. Поддерживается Markdown.",
+        "post_form.label_tags": "Теги",
+        "post_form.placeholder_tags": "python, fastapi, tutorial",
+        "post_form.hint_tags": "Список тегов через запятую",
+        "post_form.cancel": "Отмена",
+        "post_form.save_draft": "Сохранить черновик",
+        "post_form.update_post": "Обновить статью",
+        "post_form.publish_post": "Опубликовать статью",
+        "profile.title": "Профиль пользователя",
+        "profile.email": "Email:",
+        "profile.not_provided": "Не указан",
+        "profile.user_id": "ID пользователя:",
+        "profile.name": "Имя:",
+        "profile.back_home": "На главную",
+        "profile.new_post": "Новая статья",
+        "about.title": "О нас",
+        "about.page_title": "О нас",
+        "about.description": "Современный блог на FastAPI с архитектурой Domain-Driven Design.",
+        "about.signed_in": "Вы вошли как {username}.",
+        "about.browsing_guest": "Вы просматриваете как гость.",
+        "about.back_home": "На главную",
+        "flash.post_published": "Статья успешно опубликована!",
+        "flash.post_saved_draft": "Статья сохранена как черновик!",
+        "flash.post_updated": "Статья успешно обновлена!",
+        "flash.post_deleted": "Статья успешно удалена!",
+        "flash.post_not_found": "Статья не найдена.",
+        "base.close_message": "Закрыть сообщение",
+        "base.default_title": "Блог",
+        "base.meta_description": "Блог — современная платформа для блогов на FastAPI",
+        "base.meta_keywords": "блог, статьи, посты, письмо",
+        "base.meta_author": "Команда блога",
+        "lang.en": "English",
+        "lang.ru": "Русский",
+        "lang.fr": "Français",
+        "lang.de": "Deutsch",
+    },
+    "fr": {
+        "nav.home": "Accueil",
+        "nav.posts": "Articles",
+        "nav.about": "À propos",
+        "header.logo": "Blog",
+        "header.profile": "Profil",
+        "header.new_post": "Nouvel article",
+        "header.sign_out": "Déconnexion",
+        "header.sign_in": "Connexion",
+        "header.toggle_menu": "Menu",
+        "header.toggle_theme": "Changer le thème",
+        "header.lang_switcher": "Langue",
+        "footer.copyright": "© 2026 Blog. Tous droits réservés.",
+        "footer.about": "À propos",
+        "footer.privacy": "Confidentialité",
+        "footer.terms": "Conditions",
+        "footer.api": "API",
+        "home.title": "Blog — Accueil",
+        "home.meta_description": "Découvrez des histoires, réflexions et expertises d'auteurs sur tous les sujets. Un blog moderne avec FastAPI.",
+        "home.meta_keywords": "blog, articles, posts, écriture, fastapi, python",
+        "home.page_title": "Derniers articles",
+        "home.page_subtitle": "Découvrez des histoires, réflexions et expertises d'auteurs sur tous les sujets.",
+        "home.write_post": "Écrire un article",
+        "home.status_published": "Publié",
+        "home.status_draft": "Brouillon",
+        "home.read_more": "Lire la suite",
+        "home.pagination_previous": "Précédent",
+        "home.pagination_next": "Suivant",
+        "home.empty_title": "Aucun article pour le moment",
+        "home.empty_description": "Soyez le premier à écrire un article !",
+        "home.empty_action": "Créer votre premier article",
+        "post.status_published": "Publié",
+        "post.status_draft": "Brouillon",
+        "post.back_to_posts": "Retour aux articles",
+        "post.edit": "Modifier",
+        "post.delete": "Supprimer",
+        "post.delete_confirm": "Êtes-vous sûr de vouloir supprimer cet article ?",
+        "post.comments": "Commentaires",
+        "post.write_comment": "Écrire un commentaire",
+        "post.comment_placeholder": "Écrivez un commentaire... Markdown est supporté.",
+        "post.replying_to": "Répondre à",
+        "post.cancel_reply": "Annuler la réponse",
+        "post.cancel": "Annuler",
+        "post.submit_comment": "Soumettre",
+        "post.reply": "Répondre",
+        "post.no_comments": "Aucun commentaire pour le moment. Soyez le premier !",
+        "post.comment_error": "Échec de l'envoi du commentaire. Veuillez réessayer.",
+        "post_form.title_edit": "Modifier l'article",
+        "post_form.title_new": "Nouvel article",
+        "post_form.page_title_edit": "Modifier l'article",
+        "post_form.page_title_new": "Créer un nouvel article",
+        "post_form.label_title": "Titre",
+        "post_form.placeholder_title": "Entrez le titre de l'article",
+        "post_form.hint_title": "Un titre accrocheur pour votre article",
+        "post_form.label_content": "Contenu",
+        "post_form.placeholder_content": "Écrivez votre article ici...",
+        "post_form.hint_content": "Le contenu principal de votre article. Markdown est supporté.",
+        "post_form.label_tags": "Tags",
+        "post_form.placeholder_tags": "python, fastapi, tutorial",
+        "post_form.hint_tags": "Liste de tags séparés par des virgules",
+        "post_form.cancel": "Annuler",
+        "post_form.save_draft": "Sauvegarder le brouillon",
+        "post_form.update_post": "Mettre à jour",
+        "post_form.publish_post": "Publier",
+        "profile.title": "Profil utilisateur",
+        "profile.email": "Email :",
+        "profile.not_provided": "Non fourni",
+        "profile.user_id": "ID utilisateur :",
+        "profile.name": "Nom :",
+        "profile.back_home": "Retour à l'accueil",
+        "profile.new_post": "Nouvel article",
+        "about.title": "À propos",
+        "about.page_title": "À propos",
+        "about.description": "Un blog moderne construit avec FastAPI et une architecture Domain-Driven Design.",
+        "about.signed_in": "Connecté en tant que {username}.",
+        "about.browsing_guest": "Vous naviguez en tant qu'invité.",
+        "about.back_home": "Retour à l'accueil",
+        "flash.post_published": "Article publié avec succès !",
+        "flash.post_saved_draft": "Article sauvegardé comme brouillon !",
+        "flash.post_updated": "Article mis à jour avec succès !",
+        "flash.post_deleted": "Article supprimé avec succès !",
+        "flash.post_not_found": "Article non trouvé.",
+        "base.close_message": "Fermer le message",
+        "base.default_title": "Blog",
+        "base.meta_description": "Blog — Une plateforme de blog moderne construite avec FastAPI",
+        "base.meta_keywords": "blog, articles, posts, écriture",
+        "base.meta_author": "Équipe du blog",
+        "lang.en": "English",
+        "lang.ru": "Русский",
+        "lang.fr": "Français",
+        "lang.de": "Deutsch",
+    },
+    "de": {
+        "nav.home": "Startseite",
+        "nav.posts": "Beiträge",
+        "nav.about": "Über uns",
+        "header.logo": "Blog",
+        "header.profile": "Profil",
+        "header.new_post": "Neuer Beitrag",
+        "header.sign_out": "Abmelden",
+        "header.sign_in": "Anmelden",
+        "header.toggle_menu": "Menü umschalten",
+        "header.toggle_theme": "Design umschalten",
+        "header.lang_switcher": "Sprache",
+        "footer.copyright": "© 2026 Blog. Alle Rechte vorbehalten.",
+        "footer.about": "Über uns",
+        "footer.privacy": "Datenschutz",
+        "footer.terms": "AGB",
+        "footer.api": "API",
+        "home.title": "Blog — Startseite",
+        "home.meta_description": "Entdecken Sie Geschichten, Gedanken und Fachwissen von Autoren zu jedem Thema. Ein moderner Blog mit FastAPI.",
+        "home.meta_keywords": "Blog, Artikel, Beiträge, Schreiben, Fastapi, Python",
+        "home.page_title": "Neueste Beiträge",
+        "home.page_subtitle": "Entdecken Sie Geschichten, Gedanken und Fachwissen von Autoren zu jedem Thema.",
+        "home.write_post": "Beitrag schreiben",
+        "home.status_published": "Veröffentlicht",
+        "home.status_draft": "Entwurf",
+        "home.read_more": "Weiterlesen",
+        "home.pagination_previous": "Zurück",
+        "home.pagination_next": "Weiter",
+        "home.empty_title": "Noch keine Beiträge",
+        "home.empty_description": "Schreiben Sie den ersten Beitrag!",
+        "home.empty_action": "Ersten Beitrag erstellen",
+        "post.status_published": "Veröffentlicht",
+        "post.status_draft": "Entwurf",
+        "post.back_to_posts": "Zurück zu den Beiträgen",
+        "post.edit": "Bearbeiten",
+        "post.delete": "Löschen",
+        "post.delete_confirm": "Sind Sie sicher, dass Sie diesen Beitrag löschen möchten?",
+        "post.comments": "Kommentare",
+        "post.write_comment": "Kommentar schreiben",
+        "post.comment_placeholder": "Schreiben Sie einen Kommentar... Markdown wird unterstützt.",
+        "post.replying_to": "Antwort an",
+        "post.cancel_reply": "Antwort abbrechen",
+        "post.cancel": "Abbrechen",
+        "post.submit_comment": "Absenden",
+        "post.reply": "Antworten",
+        "post.no_comments": "Noch keine Kommentare. Seien Sie der Erste!",
+        "post.comment_error": "Kommentar konnte nicht gesendet werden. Bitte versuchen Sie es erneut.",
+        "post_form.title_edit": "Beitrag bearbeiten",
+        "post_form.title_new": "Neuer Beitrag",
+        "post_form.page_title_edit": "Beitrag bearbeiten",
+        "post_form.page_title_new": "Neuen Beitrag erstellen",
+        "post_form.label_title": "Titel",
+        "post_form.placeholder_title": "Geben Sie den Beitragstitel ein",
+        "post_form.hint_title": "Ein eingängiger Titel für Ihren Beitrag",
+        "post_form.label_content": "Inhalt",
+        "post_form.placeholder_content": "Schreiben Sie Ihren Beitrag hier...",
+        "post_form.hint_content": "Der Hauptinhalt Ihres Beitrags. Markdown wird unterstützt.",
+        "post_form.label_tags": "Tags",
+        "post_form.placeholder_tags": "python, fastapi, tutorial",
+        "post_form.hint_tags": "Kommagetrennte Tag-Liste",
+        "post_form.cancel": "Abbrechen",
+        "post_form.save_draft": "Als Entwurf speichern",
+        "post_form.update_post": "Beitrag aktualisieren",
+        "post_form.publish_post": "Beitrag veröffentlichen",
+        "profile.title": "Benutzerprofil",
+        "profile.email": "E-Mail:",
+        "profile.not_provided": "Nicht angegeben",
+        "profile.user_id": "Benutzer-ID:",
+        "profile.name": "Name:",
+        "profile.back_home": "Zurück zur Startseite",
+        "profile.new_post": "Neuer Beitrag",
+        "about.title": "Über uns",
+        "about.page_title": "Über uns",
+        "about.description": "Ein moderner Blog, erstellt mit FastAPI und Domain-Driven Design Architektur.",
+        "about.signed_in": "Angemeldet als {username}.",
+        "about.browsing_guest": "Sie surfen als Gast.",
+        "about.back_home": "Zurück zur Startseite",
+        "flash.post_published": "Beitrag erfolgreich veröffentlicht!",
+        "flash.post_saved_draft": "Beitrag als Entwurf gespeichert!",
+        "flash.post_updated": "Beitrag erfolgreich aktualisiert!",
+        "flash.post_deleted": "Beitrag erfolgreich gelöscht!",
+        "flash.post_not_found": "Beitrag nicht gefunden.",
+        "base.close_message": "Nachricht schließen",
+        "base.default_title": "Blog",
+        "base.meta_description": "Blog — Eine moderne Blogging-Plattform mit FastAPI",
+        "base.meta_keywords": "Blog, Artikel, Beiträge, Schreiben",
+        "base.meta_author": "Blog-Team",
+        "lang.en": "English",
+        "lang.ru": "Русский",
+        "lang.fr": "Français",
+        "lang.de": "Deutsch",
+    },
+}

app/infrastructure/i18n/translator.py

@@ -0,0 +1,78 @@
+"""Translation service for i18n support.
+
+This module provides the translation service that resolves translation keys
+to localized strings using in-memory translation dictionaries. Falls back
+from requested locale through English to the raw key.
+"""
+
+from app.infrastructure.i18n.translations import TRANSLATIONS
+
+SUPPORTED_LOCALES = frozenset({"en", "ru", "fr", "de"})
+DEFAULT_LOCALE = "en"
+
+
+class TranslationService:
+    """Service for resolving translation keys to localized strings.
+
+    Provides a singleton-like interface for translating UI strings
+    across the application. Falls back through requested locale to
+    English and finally to the raw key if no translation exists.
+
+    Attributes:
+        translations: Dictionary of locale to key to string mappings.
+    """
+
+    _instance: "TranslationService | None" = None
+
+    def __init__(self) -> None:
+        """Initialize translation service with translation data."""
+        self.translations = TRANSLATIONS
+
+    @classmethod
+    def get_instance(cls) -> "TranslationService":
+        """Get or create the singleton instance.
+
+        Returns:
+            The shared TranslationService instance.
+        """
+        if cls._instance is None:
+            cls._instance = cls()
+        return cls._instance
+
+    def get_text(self, key: str, locale: str = DEFAULT_LOCALE) -> str:
+        """Get translated text for a given key and locale.
+
+        Resolves the key through the locale chain: requested locale,
+        then English fallback, then the raw key itself.
+
+        Args:
+            key: Translation key (e.g. ``nav.home``).
+            locale: Target locale code (e.g. ``en``, ``ru``, ``fr``, ``de``).
+
+        Returns:
+            Translated string if found, otherwise the English version
+            or the key itself as last resort.
+        """
+        locale_translations = self.translations.get(locale)
+        if locale_translations is not None and key in locale_translations:
+            return locale_translations[key]
+
+        if locale != DEFAULT_LOCALE:
+            fallback = self.translations.get(DEFAULT_LOCALE, {}).get(key)
+            if fallback is not None:
+                return fallback
+
+        return key
+
+
+def _(key: str, locale: str = DEFAULT_LOCALE) -> str:
+    """Convenience function for translating a single key.
+
+    Args:
+        key: Translation key to look up.
+        locale: Target locale code.
+
+    Returns:
+        Translated string or the key itself if no translation found.
+    """
+    return TranslationService.get_instance().get_text(key, locale)

app/infrastructure/repositories/comment.py

@@ -0,0 +1,240 @@
+"""SQLAlchemy implementation of CommentRepository.
+
+This module provides the concrete implementation of CommentRepository
+using SQLAlchemy ORM for data persistence.
+"""
+
+from uuid import UUID
+
+from sqlalchemy import func, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.domain.entities.comment import Comment
+from app.domain.entities.comment_like import CommentLike
+from app.domain.repositories import CommentRepository
+from app.domain.value_objects.comment_content import CommentContent
+from app.infrastructure.database.models import CommentLikeORM, CommentORM
+
+
+class SQLAlchemyCommentRepository(CommentRepository):
+    """SQLAlchemy implementation of Comment repository.
+
+    Provides data access methods for Comment entities using SQLAlchemy ORM.
+    Handles conversion between domain entities and ORM models.
+
+    Attributes:
+        _session: SQLAlchemy async session for database operations.
+    """
+
+    def __init__(self, session: AsyncSession) -> None:
+        """Initialize repository with session.
+
+        Args:
+            session: SQLAlchemy async session instance.
+        """
+        self._session = session
+
+    def _to_domain(self, orm: CommentORM) -> Comment:
+        """Convert ORM model to domain entity.
+
+        Args:
+            orm: SQLAlchemy ORM model instance.
+
+        Returns:
+            Domain Comment entity with validated value objects.
+        """
+        return Comment(
+            id=UUID(orm.id),
+            post_id=UUID(orm.post_id),
+            author_id=orm.author_id,
+            content=CommentContent(orm.content),
+            parent_id=UUID(orm.parent_id) if orm.parent_id else None,
+            like_count=orm.like_count,
+            created_at=orm.created_at,
+            updated_at=orm.updated_at,
+        )
+
+    def _to_orm(self, comment: Comment) -> CommentORM:
+        """Convert domain entity to ORM model.
+
+        Args:
+            comment: Domain Comment entity.
+
+        Returns:
+            SQLAlchemy ORM model instance.
+        """
+        return CommentORM(
+            id=str(comment.id),
+            post_id=str(comment.post_id),
+            author_id=comment.author_id,
+            content=comment.content.value,
+            parent_id=str(comment.parent_id) if comment.parent_id else None,
+            like_count=comment.like_count,
+            created_at=comment.created_at,
+            updated_at=comment.updated_at,
+        )
+
+    async def get_by_id(self, entity_id: UUID) -> Comment | None:
+        """Get comment by ID.
+
+        Args:
+            entity_id: Unique identifier of the comment.
+
+        Returns:
+            Comment entity if found, None otherwise.
+        """
+        result = await self._session.execute(
+            select(CommentORM).where(CommentORM.id == str(entity_id))
+        )
+        orm = result.scalar_one_or_none()
+        return self._to_domain(orm) if orm else None
+
+    async def get_all(self) -> list[Comment]:
+        """Get all comments.
+
+        Returns:
+            List of all Comment entities.
+        """
+        result = await self._session.execute(select(CommentORM))
+        orms = result.scalars().all()
+        return [self._to_domain(orm) for orm in orms]
+
+    async def add(self, entity: Comment) -> None:
+        """Add new comment.
+
+        Args:
+            entity: Comment entity to add.
+        """
+        orm = self._to_orm(entity)
+        self._session.add(orm)
+
+    async def update(self, entity: Comment) -> None:
+        """Update existing comment.
+
+        Args:
+            entity: Comment entity with updated data.
+        """
+        result = await self._session.execute(
+            select(CommentORM).where(CommentORM.id == str(entity.id))
+        )
+        orm = result.scalar_one()
+
+        orm.content = entity.content.value
+        orm.like_count = entity.like_count
+        orm.updated_at = entity.updated_at
+
+    async def delete(self, entity_id: UUID) -> None:
+        """Delete comment by ID.
+
+        Args:
+            entity_id: Unique identifier of the comment to delete.
+        """
+        result = await self._session.execute(
+            select(CommentORM).where(CommentORM.id == str(entity_id))
+        )
+        orm = result.scalar_one_or_none()
+        if orm:
+            await self._session.delete(orm)
+
+    async def exists(self, entity_id: UUID) -> bool:
+        """Check if comment exists.
+
+        Args:
+            entity_id: Unique identifier of the comment.
+
+        Returns:
+            True if comment exists, False otherwise.
+        """
+        result = await self._session.execute(
+            select(CommentORM).where(CommentORM.id == str(entity_id))
+        )
+        return result.scalar_one_or_none() is not None
+
+    async def get_by_post(self, post_id: UUID) -> list[Comment]:
+        """Get all comments for a post, ordered by creation time.
+
+        Args:
+            post_id: UUID of the post.
+
+        Returns:
+            List of Comment entities for the post.
+        """
+        result = await self._session.execute(
+            select(CommentORM)
+            .where(CommentORM.post_id == str(post_id))
+            .order_by(CommentORM.created_at.asc())
+        )
+        orms = result.scalars().all()
+        return [self._to_domain(orm) for orm in orms]
+
+    async def count_by_post(self, post_id: UUID) -> int:
+        """Get comment count for a post.
+
+        Args:
+            post_id: UUID of the post.
+
+        Returns:
+            Number of comments on the post.
+        """
+        result = await self._session.execute(
+            select(func.count()).select_from(CommentORM).where(CommentORM.post_id == str(post_id))
+        )
+        count: int = result.scalar() or 0
+        return count
+
+    async def get_like(self, comment_id: UUID, liked_by: str) -> CommentLike | None:
+        """Get a like by comment and user.
+
+        Args:
+            comment_id: UUID of the comment.
+            liked_by: User ID.
+
+        Returns:
+            CommentLike if found, None otherwise.
+        """
+        result = await self._session.execute(
+            select(CommentLikeORM).where(
+                CommentLikeORM.comment_id == str(comment_id),
+                CommentLikeORM.liked_by == liked_by,
+            )
+        )
+        orm = result.scalar_one_or_none()
+        if not orm:
+            return None
+        return CommentLike(
+            id=UUID(orm.id),
+            comment_id=UUID(orm.comment_id),
+            liked_by=orm.liked_by,
+            created_at=orm.created_at,
+        )
+
+    async def add_like(self, like: CommentLike) -> None:
+        """Add a new like to a comment.
+
+        Args:
+            like: CommentLike entity to add.
+        """
+        orm = CommentLikeORM(
+            id=str(like.id),
+            comment_id=str(like.comment_id),
+            liked_by=like.liked_by,
+            created_at=like.created_at,
+        )
+        self._session.add(orm)
+
+    async def remove_like(self, comment_id: UUID, liked_by: str) -> None:
+        """Remove a like from a comment by user.
+
+        Args:
+            comment_id: UUID of the comment.
+            liked_by: User ID.
+        """
+        result = await self._session.execute(
+            select(CommentLikeORM).where(
+                CommentLikeORM.comment_id == str(comment_id),
+                CommentLikeORM.liked_by == liked_by,
+            )
+        )
+        orm = result.scalar_one_or_none()
+        if orm:
+            await self._session.delete(orm)

app/infrastructure/repositories/post.py

@@ -10,9 +10,10 @@ from sqlalchemy import or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.domain.entities import Post
+from app.domain.entities.like import PostLike
 from app.domain.repositories import PostRepository
 from app.domain.value_objects import Content, Slug, Title
-from app.infrastructure.database.models import PostORM
+from app.infrastructure.database.models import PostLikeORM, PostORM
 
 
 class SQLAlchemyPostRepository(PostRepository):
@@ -53,6 +54,7 @@ class SQLAlchemyPostRepository(PostRepository):
             slug=Slug(orm.slug),
             author_id=orm.author_id,
             published=orm.published,
+            like_count=orm.like_count,
             tags=orm.tags or [],
             created_at=orm.created_at,
             updated_at=orm.updated_at,
@@ -74,6 +76,7 @@ class SQLAlchemyPostRepository(PostRepository):
             slug=post.slug.value,
             author_id=post.author_id,
             published=post.published,
+            like_count=post.like_count,
             tags=post.tags,
             created_at=post.created_at,
             updated_at=post.updated_at,
@@ -124,6 +127,7 @@ class SQLAlchemyPostRepository(PostRepository):
         orm.content = entity.content.value
         orm.slug = entity.slug.value
         orm.published = entity.published
+        orm.like_count = entity.like_count
         orm.tags = entity.tags
         orm.updated_at = entity.updated_at
 
@@ -284,3 +288,60 @@ class SQLAlchemyPostRepository(PostRepository):
         result = await self._session.execute(stmt)
         orms = result.scalars().all()
         return [self._to_domain(orm) for orm in orms]
+
+    async def get_like(self, post_id: UUID, liked_by: str) -> PostLike | None:
+        """Get a like by post and user/device.
+
+        Args:
+            post_id: UUID of the post.
+            liked_by: User ID or device ID.
+
+        Returns:
+            PostLike if found, None otherwise.
+        """
+        result = await self._session.execute(
+            select(PostLikeORM).where(
+                PostLikeORM.post_id == str(post_id),
+                PostLikeORM.liked_by == liked_by,
+            )
+        )
+        orm = result.scalar_one_or_none()
+        if not orm:
+            return None
+        return PostLike(
+            id=UUID(orm.id),
+            post_id=UUID(orm.post_id),
+            liked_by=orm.liked_by,
+            created_at=orm.created_at,
+        )
+
+    async def add_like(self, like: PostLike) -> None:
+        """Add a new like.
+
+        Args:
+            like: PostLike entity to add.
+        """
+        orm = PostLikeORM(
+            id=str(like.id),
+            post_id=str(like.post_id),
+            liked_by=like.liked_by,
+            created_at=like.created_at,
+        )
+        self._session.add(orm)
+
+    async def remove_like(self, post_id: UUID, liked_by: str) -> None:
+        """Remove a like by post and user/device.
+
+        Args:
+            post_id: UUID of the post.
+            liked_by: User ID or device ID.
+        """
+        result = await self._session.execute(
+            select(PostLikeORM).where(
+                PostLikeORM.post_id == str(post_id),
+                PostLikeORM.liked_by == liked_by,
+            )
+        )
+        orm = result.scalar_one_or_none()
+        if orm:
+            await self._session.delete(orm)

app/main.py

@@ -28,6 +28,7 @@ from app.presentation.web import auth_router
 from app.presentation.web import router as web_router
 from app.presentation.web.error_handlers import register_error_handlers
 from app.presentation.web.flash import setup_flash_manager
+from app.presentation.web.locale import setup_locale_manager
 
 
 @asynccontextmanager
@@ -86,6 +87,15 @@ def app_factory() -> FastAPI:
             request.state.flash_manager.set_cookie(response)
         return response
 
+    @app.middleware("http")
+    async def locale_middleware(
+        request: Request, call_next: Callable[[Request], Awaitable[Response]]
+    ) -> Response:
+        """Middleware to detect and set locale for each request."""
+        await setup_locale_manager(request)
+        response = await call_next(request)
+        return response
+
     app.add_middleware(
         CORSMiddleware,
         allow_origins=["*"],

app/presentation/api/deps.py

@@ -11,11 +11,16 @@ from fastapi import Depends, Request
 from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
 
 from app.application import (
+    CreateCommentUseCase,
     CreatePostUseCase,
+    DeleteCommentUseCase,
     DeletePostUseCase,
     GetPostUseCase,
+    ListCommentsUseCase,
     ListPostsUseCase,
     PublishPostUseCase,
+    ToggleCommentLikeUseCase,
+    TogglePostLikeUseCase,
     UpdatePostUseCase,
 )
 from app.domain.exceptions import ForbiddenException, UnauthorizedException
@@ -28,6 +33,12 @@ UpdatePostDep = FromDishka[UpdatePostUseCase]
 DeletePostDep = FromDishka[DeletePostUseCase]
 ListPostsDep = FromDishka[ListPostsUseCase]
 PublishPostDep = FromDishka[PublishPostUseCase]
+ToggleLikeDep = FromDishka[TogglePostLikeUseCase]
+
+CreateCommentDep = FromDishka[CreateCommentUseCase]
+DeleteCommentDep = FromDishka[DeleteCommentUseCase]
+ListCommentsDep = FromDishka[ListCommentsUseCase]
+ToggleCommentLikeDep = FromDishka[ToggleCommentLikeUseCase]
 
 security = HTTPBearer(auto_error=False)
 

app/presentation/api/v1/__init__.py

@@ -6,7 +6,9 @@ all v1 endpoint routers.
 
 from fastapi import APIRouter
 
+from app.presentation.api.v1.comments import router as comments_router
 from app.presentation.api.v1.posts import router as posts_router
 
 router = APIRouter(prefix="/v1")
 router.include_router(posts_router)
+router.include_router(comments_router)

app/presentation/api/v1/comments.py

@@ -0,0 +1,131 @@
+"""Comments API routes.
+
+This module defines FastAPI routes for comment operations including
+CRUD and like/unlike toggle.
+"""
+
+from uuid import UUID
+
+from dishka.integrations.fastapi import DishkaRoute
+from fastapi import APIRouter, status
+
+from app.presentation.api.deps import (
+    CreateCommentDep,
+    CurrentRoleDep,
+    CurrentUserDep,
+    DeleteCommentDep,
+    ListCommentsDep,
+    ToggleCommentLikeDep,
+)
+from app.presentation.schemas import (
+    CommentCreateSchema,
+    CommentLikeResponseSchema,
+    CommentResponseSchema,
+)
+
+router = APIRouter(tags=["comments"], route_class=DishkaRoute)
+
+
+@router.post(
+    "/posts/{post_id}/comments",
+    response_model=CommentResponseSchema,
+    status_code=status.HTTP_201_CREATED,
+    summary="Create a comment on a post",
+)
+async def create_comment(
+    post_id: UUID,
+    schema: CommentCreateSchema,
+    use_case: CreateCommentDep,
+    current_user_id: CurrentUserDep,
+) -> CommentResponseSchema:
+    """Create a comment on a blog post.
+
+    Args:
+        post_id: UUID of the post to comment on.
+        schema: Comment creation data.
+        use_case: CreateCommentUseCase dependency.
+        current_user_id: Authenticated user ID.
+
+    Returns:
+        CommentResponseSchema with created comment data.
+    """
+    result = await use_case.execute(
+        post_id=post_id,
+        author_id=current_user_id,
+        content=schema.content,
+        parent_id=schema.parent_id,
+    )
+    return CommentResponseSchema(**result.__dict__)
+
+
+@router.get(
+    "/posts/{post_id}/comments",
+    response_model=list[CommentResponseSchema],
+    summary="List comments for a post",
+)
+async def list_comments(
+    post_id: UUID,
+    use_case: ListCommentsDep,
+) -> list[CommentResponseSchema]:
+    """Get all comments for a blog post.
+
+    Args:
+        post_id: UUID of the post.
+        use_case: ListCommentsUseCase dependency.
+
+    Returns:
+        List of CommentResponseSchema for the post.
+    """
+    results = await use_case.execute(post_id=post_id)
+    return [CommentResponseSchema(**r.__dict__) for r in results]
+
+
+@router.delete(
+    "/comments/{comment_id}",
+    status_code=status.HTTP_204_NO_CONTENT,
+    summary="Delete a comment",
+)
+async def delete_comment(
+    comment_id: UUID,
+    use_case: DeleteCommentDep,
+    current_user_id: CurrentUserDep,
+    role: CurrentRoleDep,
+) -> None:
+    """Delete a comment.
+
+    Users can delete their own comments.
+
+    Args:
+        comment_id: UUID of the comment to delete.
+        use_case: DeleteCommentUseCase dependency.
+        current_user_id: Authenticated user ID.
+        role: Current user role.
+    """
+    await use_case.execute(comment_id=comment_id, user_id=current_user_id)
+
+
+@router.post(
+    "/comments/{comment_id}/like",
+    response_model=CommentLikeResponseSchema,
+    summary="Toggle like on a comment",
+)
+async def toggle_comment_like(
+    comment_id: UUID,
+    use_case: ToggleCommentLikeDep,
+    current_user_id: CurrentUserDep,
+) -> CommentLikeResponseSchema:
+    """Toggle like/unlike on a comment.
+
+    If the user already liked the comment, the like is removed (unlike).
+    Otherwise, a new like is added.
+
+    Args:
+        comment_id: UUID of the comment.
+        use_case: ToggleCommentLikeUseCase dependency.
+        current_user_id: Authenticated user ID.
+
+    Returns:
+        CommentLikeResponseSchema with updated like_count.
+    """
+    result = await use_case.execute(comment_id, current_user_id)
+    return CommentLikeResponseSchema(id=result.id, like_count=result.like_count)

app/presentation/api/v1/posts.py

@@ -20,6 +20,7 @@ from app.presentation.api.deps import (
     GetPostDep,
     ListPostsDep,
     PublishPostDep,
+    ToggleLikeDep,
     UpdatePostDep,
 )
 from app.presentation.schemas import (
@@ -344,3 +345,30 @@ async def unpublish_post(
     """
     result = await use_case.unpublish(post_id, current_user_id, role)
     return PostResponseSchema(**result.__dict__)
+
+
+@router.post(
+    "/{post_id}/like",
+    response_model=PostResponseSchema,
+    summary="Toggle like on a post",
+)
+async def toggle_like(
+    post_id: UUID,
+    use_case: ToggleLikeDep,
+    current_user_id: CurrentUserDep,
+) -> PostResponseSchema:
+    """Toggle like/unlike on a post.
+
+    If the user already liked the post, the like is removed (unlike).
+    Otherwise, a new like is added.
+
+    Args:
+        post_id: Unique identifier of the post.
+        use_case: TogglePostLikeUseCase dependency.
+        current_user_id: Authenticated user ID.
+
+    Returns:
+        PostResponseSchema with updated like_count.
+    """
+    result = await use_case.execute(post_id, current_user_id)
+    return PostResponseSchema(**result.__dict__)

app/presentation/schemas/__init__.py

@@ -4,6 +4,11 @@ This module re-exports all Pydantic schemas used for
 request/response validation in the API layer.
 """
 
+from app.presentation.schemas.comment import (
+    CommentCreateSchema,
+    CommentLikeResponseSchema,
+    CommentResponseSchema,
+)
 from app.presentation.schemas.post import (
     PostBaseSchema,
     PostCreateSchema,
@@ -22,4 +27,7 @@ __all__ = [
     "PostListResponseSchema",
     "PostSearchSchema",
     "PostPublishSchema",
+    "CommentCreateSchema",
+    "CommentResponseSchema",
+    "CommentLikeResponseSchema",
 ]

app/presentation/schemas/comment.py

@@ -0,0 +1,58 @@
+"""Pydantic schemas for comments.
+
+This module defines Pydantic models for comment request/response
+validation in the API layer.
+"""
+
+from datetime import datetime
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+
+class CommentCreateSchema(BaseModel):
+    """Schema for creating a comment.
+
+    Attributes:
+        content: Comment text content (Markdown supported).
+        parent_id: Optional parent comment ID for replies.
+    """
+
+    content: str = Field(..., min_length=1, max_length=5000, description="Comment content")
+    parent_id: UUID | None = Field(default=None, description="Parent comment ID for replies")
+
+
+class CommentResponseSchema(BaseModel):
+    """Schema for comment response.
+
+    Attributes:
+        id: Unique comment identifier.
+        post_id: UUID of the parent post.
+        author_id: Comment author identifier.
+        content: Comment content text.
+        parent_id: Optional parent comment ID.
+        like_count: Number of likes on this comment.
+        created_at: Creation timestamp.
+        updated_at: Last update timestamp.
+    """
+
+    id: UUID
+    post_id: UUID
+    author_id: str
+    content: str
+    parent_id: UUID | None = None
+    like_count: int = 0
+    created_at: datetime | None = None
+    updated_at: datetime | None = None
+
+
+class CommentLikeResponseSchema(BaseModel):
+    """Schema for comment like response.
+
+    Attributes:
+        id: Comment identifier.
+        like_count: Updated like count.
+    """
+
+    id: UUID
+    like_count: int

app/presentation/schemas/post.py

@@ -81,6 +81,7 @@ class PostResponseSchema(BaseModel):
     slug: str
     author_id: str
     published: bool
+    like_count: int = 0
     tags: list[str]
     created_at: datetime
     updated_at: datetime

app/presentation/templates/base.html

@@ -1,11 +1,11 @@
 <!DOCTYPE html>
-<html lang="en" data-testid="html-root">
+<html lang="{{ current_locale }}" data-testid="html-root">
 <head>
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
-    <meta name="description" content="{% block meta_description %}Blog - A modern blogging platform built with FastAPI{% endblock %}">
-    <meta name="keywords" content="{% block meta_keywords %}blog, articles, posts, writing{% endblock %}">
-    <meta name="author" content="{% block meta_author %}Blog Team{% endblock %}">
+    <meta name="description" content="{% block meta_description %}{{ _('base.meta_description', current_locale) }}{% endblock %}">
+    <meta name="keywords" content="{% block meta_keywords %}{{ _('base.meta_keywords', current_locale) }}{% endblock %}">
+    <meta name="author" content="{% block meta_author %}{{ _('base.meta_author', current_locale) }}{% endblock %}">
     <meta name="robots" content="{% block meta_robots %}index, follow{% endblock %}">
     
     <!-- Canonical URL -->
@@ -30,7 +30,7 @@
     <link rel="icon" type="image/svg+xml" href="/static/images/favicon.svg">
     <link rel="alternate icon" href="/static/images/favicon.ico">
     
-    <title data-testid="page-title">{% block title %}Blog{% endblock %}</title>
+    <title data-testid="page-title">{% block title %}{{ _('base.default_title', current_locale) }}{% endblock %}</title>
     
     <link rel="stylesheet" href="/static/css/themes/theme-light.css" data-testid="theme-light-stylesheet">
     <link rel="stylesheet" href="/static/css/themes/theme-dark.css" data-testid="theme-dark-stylesheet">
@@ -51,7 +51,7 @@
         {% for msg in flash_messages %}
         <div class="flash-message flash-{{ msg.category }}" data-testid="flash-message-{{ msg.category }}" role="alert">
             <span class="flash-text" data-testid="flash-text">{{ msg.message }}</span>
-            <button type="button" class="flash-close" data-testid="flash-close" aria-label="Close message">&times;</button>
+            <button type="button" class="flash-close" data-testid="flash-close" aria-label="{{ _('base.close_message', current_locale) }}">&times;</button>
         </div>
         {% endfor %}
     </div>

app/presentation/templates/pages/about.html

@@ -1,26 +1,26 @@
 {% extends "base.html" %}
 
-{% block title %}About - Blog{% endblock %}
-{% block meta_description %}A modern blog built with FastAPI and DDD architecture.{% endblock %}
+{% block title %}{{ _('about.title', current_locale) }} - {{ _('base.default_title', current_locale) }}{% endblock %}
+{% block meta_description %}{{ _('about.description', current_locale) }}{% endblock %}
 
 {% block content %}
 <div class="page-header" data-testid="page-header-about">
-    <h1 class="page-title" data-testid="page-title-about">About</h1>
+    <h1 class="page-title" data-testid="page-title-about">{{ _('about.page_title', current_locale) }}</h1>
 </div>
 
 <div class="card" data-testid="about-card">
     <div class="card-body" data-testid="about-card-body">
         <p data-testid="about-description">
-            A modern blog built with FastAPI and Domain-Driven Design architecture.
+            {{ _('about.description', current_locale) }}
         </p>
 
         <div class="divider" data-testid="about-divider"></div>
 
         <p data-testid="about-user">
             {% if user %}
-            Signed in as <strong>{{ user.username }}</strong>.
+            {{ _('about.signed_in', current_locale).format(username=user.username) }}
             {% else %}
-            You are browsing as a guest.
+            {{ _('about.browsing_guest', current_locale) }}
             {% endif %}
         </p>
     </div>
@@ -30,7 +30,7 @@
             <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                 <path d="M10 12L6 8L10 4" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
             </svg>
-            Back to Home
+            {{ _('about.back_home', current_locale) }}
         </a>
     </div>
 </div>

app/presentation/templates/pages/index.html

@@ -1,29 +1,29 @@
 {% extends "base.html" %}
 
-{% block title %}Blog - Home{% endblock %}
-{% block meta_description %}Discover stories, thinking, and expertise from writers on any topic. A modern blog built with FastAPI.{% endblock %}
-{% block meta_keywords %}blog, articles, posts, writing, fastapi, python{% endblock %}
+{% block title %}{{ _('home.title', current_locale) }}{% endblock %}
+{% block meta_description %}{{ _('home.meta_description', current_locale) }}{% endblock %}
+{% block meta_keywords %}{{ _('home.meta_keywords', current_locale) }}{% endblock %}
 
 {% block og_type %}website{% endblock %}
-{% block og_title %}Blog - Home{% endblock %}
-{% block og_description %}Discover stories, thinking, and expertise from writers on any topic.{% endblock %}
+{% block og_title %}{{ _('home.title', current_locale) }}{% endblock %}
+{% block og_description %}{{ _('home.meta_description', current_locale) }}{% endblock %}
 
-{% block twitter_title %}Blog - Home{% endblock %}
-{% block twitter_description %}Discover stories, thinking, and expertise from writers on any topic.{% endblock %}
+{% block twitter_title %}{{ _('home.title', current_locale) }}{% endblock %}
+{% block twitter_description %}{{ _('home.meta_description', current_locale) }}{% endblock %}
 
 {% block content %}
 <section class="page-header" data-testid="page-header-home">
     <div class="page-header-flex">
         <div data-testid="page-header-content">
-            <h1 class="page-title" data-testid="page-title-home">Latest Posts</h1>
-            <p class="page-subtitle" data-testid="page-subtitle-home">Discover stories, thinking, and expertise from writers on any topic.</p>
+            <h1 class="page-title" data-testid="page-title-home">{{ _('home.page_title', current_locale) }}</h1>
+            <p class="page-subtitle" data-testid="page-subtitle-home">{{ _('home.page_subtitle', current_locale) }}</p>
         </div>
         {% if can_create %}
         <a href="/web/posts/new" class="btn btn-primary" data-testid="btn-create-post-header">
             <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                 <path d="M8 2v12M2 8h12" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
             </svg>
-            Write a Post
+            {{ _('home.write_post', current_locale) }}
         </a>
         {% endif %}
     </div>
@@ -38,9 +38,9 @@
                 <a href="/web/posts/{{ post.slug }}" data-testid="post-title-link-{{ post.id }}">{{ post.title }}</a>
             </h2>
             {% if post.published %}
-            <span class="badge badge-success" data-testid="post-status-{{ post.id }}">Published</span>
+            <span class="badge badge-success" data-testid="post-status-{{ post.id }}">{{ _('home.status_published', current_locale) }}</span>
             {% else %}
-            <span class="badge" data-testid="post-status-{{ post.id }}">Draft</span>
+            <span class="badge" data-testid="post-status-{{ post.id }}">{{ _('home.status_draft', current_locale) }}</span>
             {% endif %}
         </div>
         
@@ -52,6 +52,12 @@
             <span class="post-card-meta-item" data-testid="post-date-{{ post.id }}">
                 {{ post.created_at.strftime('%B %d, %Y') }}
             </span>
+            <span class="post-card-meta-item" data-testid="like-count-{{ post.id }}">
+                👍 {{ post.like_count }}
+            </span>
+            <span class="post-card-meta-item" data-testid="comment-count-{{ post.id }}">
+                💬 {{ post.comment_count }}
+            </span>
         </div>
         
         <div class="post-card-content" data-testid="post-content-preview-{{ post.id }}">
@@ -65,7 +71,7 @@
                 {% endfor %}
             </div>
             <a href="/web/posts/{{ post.slug }}" class="btn btn-sm" data-testid="btn-read-more-{{ post.id }}">
-                Read more
+                {{ _('home.read_more', current_locale) }}
                 <svg width="14" height="14" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-left: 0.25rem;">
                     <path d="M6 12L10 8L6 4" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
@@ -77,26 +83,26 @@
 
 <nav class="pagination" data-testid="pagination" aria-label="Pagination">
     {% if has_prev %}
-    <a href="{{ request.url.path }}?page={{ current_page - 1 }}" class="pagination-item" data-testid="pagination-prev">Previous</a>
+    <a href="{{ request.url.path }}?page={{ current_page - 1 }}" class="pagination-item" data-testid="pagination-prev">{{ _('home.pagination_previous', current_locale) }}</a>
     {% else %}
-    <span class="pagination-item disabled" data-testid="pagination-prev">Previous</span>
+    <span class="pagination-item disabled" data-testid="pagination-prev">{{ _('home.pagination_previous', current_locale) }}</span>
     {% endif %}
     
     <span class="pagination-item active" data-testid="pagination-current">{{ current_page }}</span>
     
     {% if has_next %}
-    <a href="{{ request.url.path }}?page={{ current_page + 1 }}" class="pagination-item" data-testid="pagination-next">Next</a>
+    <a href="{{ request.url.path }}?page={{ current_page + 1 }}" class="pagination-item" data-testid="pagination-next">{{ _('home.pagination_next', current_locale) }}</a>
     {% else %}
-    <span class="pagination-item disabled" data-testid="pagination-next">Next</span>
+    <span class="pagination-item disabled" data-testid="pagination-next">{{ _('home.pagination_next', current_locale) }}</span>
     {% endif %}
 </nav>
 
 {% else %}
 <div class="empty-state" data-testid="empty-state">
     <div class="empty-state-icon" data-testid="empty-state-icon">📝</div>
-    <h3 data-testid="empty-state-title">No posts yet</h3>
-    <p data-testid="empty-state-description">Be the first to write a post!</p>
-    <a href="/web/posts/new" class="btn btn-primary" data-testid="btn-create-first-post">Create your first post</a>
+    <h3 data-testid="empty-state-title">{{ _('home.empty_title', current_locale) }}</h3>
+    <p data-testid="empty-state-description">{{ _('home.empty_description', current_locale) }}</p>
+    <a href="/web/posts/new" class="btn btn-primary" data-testid="btn-create-first-post">{{ _('home.empty_action', current_locale) }}</a>
 </div>
 {% endif %}
 {% endblock %}

app/presentation/templates/pages/post_detail.html

@@ -19,7 +19,7 @@
 <article class="post-detail" data-testid="post-detail">
     <header class="post-detail-header" data-testid="post-detail-header">
         <h1 class="post-detail-title" data-testid="post-detail-title">{{ post.title }}</h1>
-        
+
         <div class="post-detail-meta" data-testid="post-detail-meta">
             <span class="post-card-meta-item" data-testid="post-detail-author">
                 <span class="avatar avatar-sm" data-testid="post-detail-author-avatar">{{ post.author_id[0]|upper }}</span>
@@ -29,51 +29,61 @@
                 {{ post.created_at.strftime('%B %d, %Y') }}
             </span>
             {% if post.published %}
-            <span class="badge badge-success" data-testid="post-detail-status">Published</span>
+            <span class="badge badge-success" data-testid="post-detail-status">{{ _('post.status_published', current_locale) }}</span>
             {% else %}
-            <span class="badge" data-testid="post-detail-status">Draft</span>
+            <span class="badge" data-testid="post-detail-status">{{ _('post.status_draft', current_locale) }}</span>
             {% endif %}
+            <span class="post-card-meta-item" data-testid="post-detail-like-count">
+                <button id="like-button" class="btn-like" data-testid="like-button"
+                        data-post-slug="{{ post.slug }}"
+                        data-liked="false">
+                    👍 <span id="like-count">{{ post.like_count }}</span>
+                </button>
+            </span>
+            <span class="post-card-meta-item" data-testid="post-detail-comment-count">
+                💬 {{ post.comment_count }}
+            </span>
         </div>
     </header>
-    
+
     <div class="post-detail-content markdown-body" data-testid="post-detail-content">
         {{ post.content|markdown|safe }}
     </div>
-    
+
     <footer class="post-detail-footer" data-testid="post-detail-footer">
         <div class="post-detail-tags" data-testid="post-detail-tags">
             {% for tag in post.tags %}
             <span class="tag" data-testid="post-detail-tag-{{ loop.index }}">{{ tag }}</span>
             {% endfor %}
         </div>
-        
+
         <div class="divider" data-testid="post-detail-divider"></div>
-        
+
         <div class="flex justify-between items-center" data-testid="post-detail-actions">
             <a href="/" class="btn" data-testid="btn-back-to-posts">
                 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                     <path d="M10 12L6 8L10 4" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
-                Back to posts
+                {{ _('post.back_to_posts', current_locale) }}
             </a>
-            
+
             {% if can_edit or can_delete %}
             <div class="flex gap-2" data-testid="post-detail-edit-actions">
                 {% if can_edit %}
-                <a href="/web/posts/{{ post.slug }}/edit" class="btn" data-testid="btn-edit-post">
-                    <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
-                        <path d="M11 2L14 5M2 14L3 10L12 1L15 4L6 13L2 14Z" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
-                    </svg>
-                    Edit
-                </a>
+                    <a href="/web/posts/{{ post.slug }}/edit" class="btn" data-testid="btn-edit-post">
+                        <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
+                            <path d="M11 2L14 5M2 14L3 10L12 1L15 4L6 13L2 14Z" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
+                        </svg>
+                        {{ _('post.edit', current_locale) }}
+                    </a>
                 {% endif %}
                 {% if can_delete %}
                 <form action="/web/posts/{{ post.slug }}/delete" method="POST" style="display: inline;" data-testid="form-delete-post">
-                    <button type="submit" class="btn btn-danger" data-testid="btn-delete-post" onclick="return confirm('Are you sure you want to delete this post?');">
+                    <button type="submit" class="btn btn-danger" data-testid="btn-delete-post" onclick="return confirm('{{ _('post.delete_confirm', current_locale) }}');">
                         <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                             <path d="M2 4h12M6 4V2a2 2 0 012-2h0a2 2 0 012 2v2m3 0v10a2 2 0 01-2 2H5a2 2 0 01-2-2V4h9z" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                         </svg>
-                        Delete
+                        {{ _('post.delete', current_locale) }}
                     </button>
                 </form>
                 {% endif %}
@@ -82,4 +92,269 @@
         </div>
     </footer>
 </article>
+
+<section class="comments-section" data-testid="comments-section">
+    <div class="comments-header" data-testid="comments-header">
+        <h2 class="comments-title" data-testid="comments-title">
+            💬 {{ _('post.comments', current_locale) }}
+            <span class="comments-count" data-testid="comments-count">({{ post.comment_count }})</span>
+        </h2>
+        {% if user %}
+        <button id="btn-show-comment-form" class="btn btn-primary" data-testid="btn-show-comment-form">
+            {{ _('post.write_comment', current_locale) }}
+        </button>
+        {% endif %}
+    </div>
+
+    {% if user %}
+    <div id="comment-form-wrapper" class="comment-form-wrapper" data-testid="comment-form-wrapper" style="display: none;">
+        <form id="comment-form" class="comment-form" data-testid="form-create-comment" data-post-slug="{{ post.slug }}">
+            <div class="form-group">
+                <textarea id="comment-content" class="form-textarea" data-testid="input-comment-content"
+                          rows="4" placeholder="{{ _('post.comment_placeholder', current_locale) }}"
+                          required minlength="1" maxlength="5000"></textarea>
+                <input type="hidden" id="comment-parent-id" name="parent_id" value="">
+                <p class="form-help" data-testid="comment-form-help" id="reply-info" style="display: none;">
+                    {{ _('post.replying_to', current_locale) }}
+                    <button type="button" class="btn-cancel-reply" data-testid="btn-cancel-reply">{{ _('post.cancel_reply', current_locale) }}</button>
+                </p>
+            </div>
+            <div class="form-actions">
+                <button type="submit" class="btn btn-primary" data-testid="submit-comment">
+                    {{ _('post.submit_comment', current_locale) }}
+                </button>
+                <button type="button" class="btn btn-cancel-comment" data-testid="btn-cancel-comment" style="display: none;">
+                    {{ _('post.cancel', current_locale) }}
+                </button>
+            </div>
+        </form>
+        <div id="comment-error" class="comment-error" data-testid="comment-error" style="display: none;"></div>
+    </div>
+    {% endif %}
+
+    {% macro render_comment(comment, depth) %}
+    <div class="comment{% if depth > 0 %} comment-reply{% endif %}" data-testid="comment-{{ comment.id }}" data-comment-id="{{ comment.id }}">
+        <div class="comment-avatar" data-testid="comment-avatar-{{ comment.id }}">
+            <span class="avatar avatar-sm">{{ comment.author_id[0]|upper }}</span>
+        </div>
+        <div class="comment-body" data-testid="comment-body-{{ comment.id }}">
+            <div class="comment-meta" data-testid="comment-meta-{{ comment.id }}">
+                <span class="comment-author" data-testid="comment-author-{{ comment.id }}">{{ comment.author_id }}</span>
+                <span class="comment-date" data-testid="comment-date-{{ comment.id }}">
+                    {% if comment.created_at %}{{ comment.created_at.strftime('%B %d, %Y') }}{% endif %}
+                </span>
+            </div>
+            <div class="comment-content" data-testid="comment-content-{{ comment.id }}">{{ comment.content }}</div>
+            <div class="comment-actions" data-testid="comment-actions-{{ comment.id }}">
+                {% if user %}
+                <button class="btn-comment-reply btn btn-sm" data-testid="btn-comment-reply-{{ comment.id }}"
+                        data-comment-id="{{ comment.id }}" data-comment-author="{{ comment.author_id }}">
+                    {{ _('post.reply', current_locale) }}
+                </button>
+                {% endif %}
+                <button class="btn-comment-like btn btn-sm" data-testid="btn-comment-like-{{ comment.id }}"
+                        data-comment-id="{{ comment.id }}">
+                    👍 <span class="comment-like-count" data-testid="comment-like-count-{{ comment.id }}">{{ comment.like_count }}</span>
+                </button>
+            </div>
+
+            {% set key = comment.id|string %}
+            {% if key in reply_comments %}
+            <div class="comment-replies" data-testid="comment-replies-{{ comment.id }}">
+                {% for child in reply_comments[key] %}
+                {{ render_comment(child, depth + 1) }}
+                {% endfor %}
+            </div>
+            {% endif %}
+        </div>
+    </div>
+    {% endmacro %}
+
+    <div class="comments-list" data-testid="comments-list">
+        {% if top_level_comments %}
+            {% for comment in top_level_comments %}
+            {{ render_comment(comment, 0) }}
+            {% endfor %}
+        {% else %}
+        <div class="comments-empty" data-testid="comments-empty">
+            <p>{{ _('post.no_comments', current_locale) }}</p>
+        </div>
+        {% endif %}
+    </div>
+</section>
+{% endblock %}
+
+{% block extra_js %}
+<script data-testid="comment-script">
+document.addEventListener('DOMContentLoaded', function() {
+    var likeButton = document.getElementById('like-button');
+    if (likeButton) {
+        likeButton.addEventListener('click', function() {
+            var slug = this.getAttribute('data-post-slug');
+            var countSpan = document.getElementById('like-count');
+
+            fetch('/web/posts/' + slug + '/like', {
+                method: 'POST',
+                headers: {
+                    'Accept': 'application/json'
+                }
+            })
+            .then(function(response) {
+                if (response.status === 401) {
+                    window.location.href = '/auth/dev-login';
+                    return;
+                }
+                if (!response.ok) {
+                    throw new Error('Like request failed');
+                }
+                return response.json();
+            })
+            .then(function(data) {
+                if (data && data.like_count !== undefined) {
+                    countSpan.textContent = data.like_count;
+                }
+            })
+            .catch(function(error) {
+                console.error('Like error:', error);
+            });
+        });
+    }
+
+    var showFormBtn = document.getElementById('btn-show-comment-form');
+    var formWrapper = document.getElementById('comment-form-wrapper');
+    var cancelBtn = document.querySelector('.btn-cancel-comment');
+    var commentForm = document.getElementById('comment-form');
+    var commentContent = document.getElementById('comment-content');
+    var commentParentId = document.getElementById('comment-parent-id');
+    var replyInfo = document.getElementById('reply-info');
+    var commentError = document.getElementById('comment-error');
+
+    function showCommentForm(parentId, authorName) {
+        commentParentId.value = parentId || '';
+        if (parentId && authorName) {
+            replyInfo.style.display = 'block';
+            replyInfo.innerHTML = '{{ _("post.replying_to", current_locale) }} <strong>' + authorName + '</strong> &mdash; <button type="button" class="btn-cancel-reply" id="btn-cancel-reply">{{ _("post.cancel_reply", current_locale) }}</button>';
+            document.getElementById('btn-cancel-reply').addEventListener('click', function() {
+                commentParentId.value = '';
+                replyInfo.style.display = 'none';
+            });
+        } else {
+            replyInfo.style.display = 'none';
+        }
+        formWrapper.style.display = 'block';
+        if (showFormBtn) showFormBtn.style.display = 'none';
+        if (cancelBtn) cancelBtn.style.display = 'inline-flex';
+        commentContent.focus();
+        commentError.style.display = 'none';
+    }
+
+    function hideCommentForm() {
+        formWrapper.style.display = 'none';
+        if (showFormBtn) showFormBtn.style.display = 'inline-flex';
+        if (cancelBtn) cancelBtn.style.display = 'none';
+        commentContent.value = '';
+        commentParentId.value = '';
+        replyInfo.style.display = 'none';
+        commentError.style.display = 'none';
+    }
+
+    if (showFormBtn) {
+        showFormBtn.addEventListener('click', function() {
+            showCommentForm(null, null);
+        });
+    }
+
+    if (cancelBtn) {
+        cancelBtn.addEventListener('click', hideCommentForm);
+    }
+
+    if (commentForm) {
+        commentForm.addEventListener('submit', function(e) {
+            e.preventDefault();
+
+            var content = commentContent.value.trim();
+            if (!content) return;
+
+            var slug = this.getAttribute('data-post-slug');
+            var parentId = commentParentId.value || null;
+
+            var payload = {content: content};
+            if (parentId) {
+                payload.parent_id = parentId;
+            }
+
+            fetch('/web/posts/' + slug + '/comments', {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json',
+                    'Accept': 'application/json'
+                },
+                body: JSON.stringify(payload)
+            })
+            .then(function(response) {
+                if (response.status === 401) {
+                    window.location.href = '/auth/dev-login';
+                    return;
+                }
+                if (!response.ok) {
+                    throw new Error('Comment creation failed');
+                }
+                return response.json();
+            })
+            .then(function(data) {
+                if (data) {
+                    location.reload();
+                }
+            })
+            .catch(function(error) {
+                commentError.textContent = '{{ _("post.comment_error", current_locale) }}';
+                commentError.style.display = 'block';
+                console.error('Comment error:', error);
+            });
+        });
+    }
+
+    var replyButtons = document.querySelectorAll('.btn-comment-reply');
+    replyButtons.forEach(function(btn) {
+        btn.addEventListener('click', function() {
+            var commentId = this.getAttribute('data-comment-id');
+            var author = this.getAttribute('data-comment-author');
+            showCommentForm(commentId, author);
+        });
+    });
+
+    var commentLikeButtons = document.querySelectorAll('.btn-comment-like');
+    commentLikeButtons.forEach(function(btn) {
+        btn.addEventListener('click', function() {
+            var commentId = this.getAttribute('data-comment-id');
+            var countSpan = this.querySelector('.comment-like-count');
+
+            fetch('/web/comments/' + commentId + '/like', {
+                method: 'POST',
+                headers: {
+                    'Accept': 'application/json'
+                }
+            })
+            .then(function(response) {
+                if (response.status === 401) {
+                    window.location.href = '/auth/dev-login';
+                    return;
+                }
+                if (!response.ok) {
+                    throw new Error('Comment like failed');
+                }
+                return response.json();
+            })
+            .then(function(data) {
+                if (data && data.like_count !== undefined) {
+                    countSpan.textContent = data.like_count;
+                }
+            })
+            .catch(function(error) {
+                console.error('Comment like error:', error);
+            });
+        });
+    });
+});
+</script>
 {% endblock %}

app/presentation/templates/pages/post_form.html

@@ -1,6 +1,6 @@
 {% extends "base.html" %}
 
-{% block title %}{% if is_edit %}Edit Post{% else %}New Post{% endif %} - Blog{% endblock %}
+{% block title %}{% if is_edit %}{{ _('post_form.title_edit', current_locale) }}{% else %}{{ _('post_form.title_new', current_locale) }}{% endif %} - {{ _('base.default_title', current_locale) }}{% endblock %}
 
 {% block extra_css %}
 <link rel="stylesheet" href="/static/css/easymde.min.css" data-testid="easymde-stylesheet">
@@ -9,7 +9,7 @@
 {% block content %}
 <section class="page-header" data-testid="page-header-form">
     <h1 class="page-title" data-testid="page-title-form">
-        {% if is_edit %}Edit Post{% else %}Create New Post{% endif %}
+        {% if is_edit %}{{ _('post_form.page_title_edit', current_locale) }}{% else %}{{ _('post_form.page_title_new', current_locale) }}{% endif %}
     </h1>
 </section>
 
@@ -22,7 +22,7 @@
     <div class="card-body" data-testid="form-post-body">
         <div class="form-group" data-testid="form-group-title">
             <label for="title" class="form-label form-label-required" data-testid="label-title">
-                Title
+                {{ _('post_form.label_title', current_locale) }}
             </label>
             <input
                 type="text"
@@ -30,31 +30,31 @@
                 name="title"
                 class="input input-lg"
                 value="{% if post %}{{ post.title }}{% endif %}"
-                placeholder="Enter post title"
+                placeholder="{{ _('post_form.placeholder_title', current_locale) }}"
                 required
                 data-testid="input-title"
             >
-            <span class="form-hint" data-testid="hint-title">A catchy title for your post</span>
+            <span class="form-hint" data-testid="hint-title">{{ _('post_form.hint_title', current_locale) }}</span>
         </div>
 
         <div class="form-group" data-testid="form-group-content">
             <label for="content" class="form-label form-label-required" data-testid="label-content">
-                Content
+                {{ _('post_form.label_content', current_locale) }}
             </label>
             <textarea
                 id="content"
                 name="content"
                 rows="12"
-                placeholder="Write your post content here..."
+                placeholder="{{ _('post_form.placeholder_content', current_locale) }}"
                 required
                 data-testid="textarea-content"
             >{% if post %}{{ post.content }}{% endif %}</textarea>
-            <span class="form-hint" data-testid="hint-content">The main content of your post. Markdown is supported.</span>
+            <span class="form-hint" data-testid="hint-content">{{ _('post_form.hint_content', current_locale) }}</span>
         </div>
         
         <div class="form-group" data-testid="form-group-tags">
             <label for="tags" class="form-label" data-testid="label-tags">
-                Tags
+                {{ _('post_form.label_tags', current_locale) }}
             </label>
             <input 
                 type="text" 
@@ -62,10 +62,10 @@
                 name="tags" 
                 class="input"
                 value="{% if post %}{{ post.tags|join(', ') }}{% endif %}"
-                placeholder="python, fastapi, tutorial"
+                placeholder="{{ _('post_form.placeholder_tags', current_locale) }}"
                 data-testid="input-tags"
             >
-            <span class="form-hint" data-testid="hint-tags">Comma-separated list of tags</span>
+            <span class="form-hint" data-testid="hint-tags">{{ _('post_form.hint_tags', current_locale) }}</span>
         </div>
         
     </div>
@@ -73,15 +73,15 @@
     <div class="card-footer" data-testid="form-post-footer">
         <div class="flex justify-between items-center" data-testid="form-actions">
             <a href="{% if is_edit %}/web/posts/{{ post.slug }}{% else %}/web/{% endif %}" class="btn" data-testid="btn-cancel">
-                Cancel
+                {{ _('post_form.cancel', current_locale) }}
             </a>
             
             <div class="flex gap-2" data-testid="form-submit-actions">
                 <button type="submit" name="action" value="draft" class="btn" data-testid="btn-save-draft">
-                    Save as Draft
+                    {{ _('post_form.save_draft', current_locale) }}
                 </button>
                 <button type="submit" name="action" value="publish" class="btn btn-primary" data-testid="btn-publish-post">
-                    {% if is_edit %}Update Post{% else %}Publish Post{% endif %}
+                    {% if is_edit %}{{ _('post_form.update_post', current_locale) }}{% else %}{{ _('post_form.publish_post', current_locale) }}{% endif %}
                 </button>
             </div>
         </div>
@@ -99,7 +99,7 @@
         spellChecker: false,
         status: false,
         minHeight: '300px',
-        placeholder: 'Write your post content here...',
+        placeholder: '{{ _('post_form.placeholder_content', current_locale) }}',
         toolbar: [
             'bold', 'italic', 'heading', '|',
             'code', 'quote', 'unordered-list', 'ordered-list', '|',

app/presentation/templates/pages/profile.html

@@ -4,7 +4,7 @@
 
 {% block content %}
 <div class="page-header" data-testid="page-header-profile">
-    <h1 class="page-title" data-testid="page-title-profile">User Profile</h1>
+    <h1 class="page-title" data-testid="page-title-profile">{{ _('profile.title', current_locale) }}</h1>
 </div>
 
 <div class="card" data-testid="profile-card">
@@ -25,18 +25,18 @@
 
         <div class="profile-details" data-testid="profile-details">
             <div class="profile-field" data-testid="profile-field-email">
-                <span class="profile-label" data-testid="profile-label-email">Email:</span>
-                <span class="profile-value" data-testid="profile-value-email">{{ user.email or 'Not provided' }}</span>
+                <span class="profile-label" data-testid="profile-label-email">{{ _('profile.email', current_locale) }}</span>
+                <span class="profile-value" data-testid="profile-value-email">{{ user.email or _('profile.not_provided', current_locale) }}</span>
             </div>
 
             <div class="profile-field" data-testid="profile-field-userid">
-                <span class="profile-label" data-testid="profile-label-userid">User ID:</span>
+                <span class="profile-label" data-testid="profile-label-userid">{{ _('profile.user_id', current_locale) }}</span>
                 <span class="profile-value" data-testid="profile-value-userid">{{ user.user_id }}</span>
             </div>
 
             {% if user.first_name or user.last_name %}
             <div class="profile-field" data-testid="profile-field-name">
-                <span class="profile-label" data-testid="profile-label-name">Name:</span>
+                <span class="profile-label" data-testid="profile-label-name">{{ _('profile.name', current_locale) }}</span>
                 <span class="profile-value" data-testid="profile-value-name">
                     {{ user.first_name or '' }} {{ user.last_name or '' }}
                 </span>
@@ -51,7 +51,7 @@
                 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                     <path d="M10 12L6 8L10 4" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
-                Back to Home
+                {{ _('profile.back_home', current_locale) }}
             </a>
 
             {% if can_create %}
@@ -59,7 +59,7 @@
                 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                     <path d="M8 2v12M2 8h12" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
-                New Post
+                {{ _('profile.new_post', current_locale) }}
             </a>
             {% endif %}
         </div>

app/presentation/templates/partials/footer.html

@@ -1,14 +1,14 @@
 <footer class="site-footer" data-testid="site-footer">
     <div class="container" data-testid="footer-container">
         <div class="footer-copyright" data-testid="footer-copyright">
-            <span data-testid="copyright-text">© 2026 Blog. All rights reserved.</span>
+            <span data-testid="copyright-text">{{ _('footer.copyright', current_locale) }}</span>
         </div>
         
         <nav class="footer-links" data-testid="footer-nav" aria-label="Footer navigation">
-            <a href="/about" class="footer-link" data-testid="footer-link-about">About</a>
-            <a href="/privacy" class="footer-link" data-testid="footer-link-privacy">Privacy</a>
-            <a href="/terms" class="footer-link" data-testid="footer-link-terms">Terms</a>
-            <a href="/api/docs" class="footer-link" data-testid="footer-link-api">API</a>
+            <a href="/about" class="footer-link" data-testid="footer-link-about">{{ _('footer.about', current_locale) }}</a>
+            <a href="/privacy" class="footer-link" data-testid="footer-link-privacy">{{ _('footer.privacy', current_locale) }}</a>
+            <a href="/terms" class="footer-link" data-testid="footer-link-terms">{{ _('footer.terms', current_locale) }}</a>
+            <a href="/api/docs" class="footer-link" data-testid="footer-link-api">{{ _('footer.api', current_locale) }}</a>
         </nav>
     </div>
 </footer>

app/presentation/templates/partials/header.html

@@ -5,7 +5,7 @@
                 <rect width="32" height="32" rx="6" fill="var(--color-primary)"/>
                 <path d="M8 12h16M8 16h12M8 20h8" stroke="white" stroke-width="2" stroke-linecap="round"/>
             </svg>
-            <span data-testid="logo-text">Blog</span>
+            <span data-testid="logo-text">{{ _('header.logo', current_locale) }}</span>
         </a>
 
         {% include "partials/nav.html" %}
@@ -15,7 +15,7 @@
                 type="button"
                 class="mobile-menu-btn"
                 data-testid="mobile-menu-toggle"
-                aria-label="Toggle menu"
+                aria-label="{{ _('header.toggle_menu', current_locale) }}"
                 aria-expanded="false"
                 aria-controls="mobile-nav"
             >
@@ -30,8 +30,8 @@
                 type="button" 
                 class="theme-toggle" 
                 data-testid="theme-toggle"
-                aria-label="Toggle dark mode"
-                title="Toggle dark mode"
+                aria-label="{{ _('header.toggle_theme', current_locale) }}"
+                title="{{ _('header.toggle_theme', current_locale) }}"
             >
                 <svg width="20" height="20" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg" data-testid="theme-light-icon" style="display: none;">
                     <path d="M10 2v2M10 16v2M4.22 4.22l1.42 1.42M14.36 14.36l1.42 1.42M2 10h2M16 10h2M4.22 15.78l1.42-1.42M14.36 5.64l1.42-1.42" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
@@ -41,7 +41,27 @@
                     <path d="M17.293 13.293A8 8 0 116.707 2.707a8.003 8.003 0 0010.586 10.586z" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
             </button>
-            
+
+            <div class="lang-switcher" data-testid="lang-switcher">
+                <button
+                    type="button"
+                    class="lang-switcher-toggle"
+                    data-testid="lang-switcher-toggle"
+                    aria-haspopup="true"
+                    aria-expanded="false"
+                    title="{{ _('header.lang_switcher', current_locale) }}"
+                >
+                    <span data-testid="current-lang-code">{{ current_locale|upper }}</span>
+                </button>
+                <div class="lang-switcher-dropdown" data-testid="lang-switcher-dropdown">
+                    {% for code in ('en', 'ru', 'fr', 'de') %}
+                    <a href="/web/lang/{{ code }}" class="lang-switcher-item {% if code == current_locale %}active{% endif %}" data-testid="lang-option-{{ code }}">
+                        {{ _('lang.' + code, current_locale) }}
+                    </a>
+                    {% endfor %}
+                </div>
+            </div>
+
             {% if user %}
             <div class="user-menu" data-testid="user-menu">
                 <button 
@@ -63,14 +83,14 @@
                             <circle cx="8" cy="6" r="3" stroke="currentColor" stroke-width="2"/>
                             <path d="M2 14c0-3 3-5 6-5s6 2 6 5" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
                         </svg>
-                        Profile
+                        {{ _('header.profile', current_locale) }}
                     </a>
                     {% if can_create %}
                     <a href="/web/posts/new" class="user-menu-item" data-testid="user-menu-new-post">
                         <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                             <path d="M8 2v12M2 8h12" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                         </svg>
-                        New Post
+                        {{ _('header.new_post', current_locale) }}
                     </a>
                     {% endif %}
                     <div class="user-menu-divider" data-testid="user-menu-divider"></div>
@@ -78,13 +98,13 @@
                         <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                             <path d="M10 12h2a2 2 0 002-2V6a2 2 0 00-2-2h-2M6 12l-3-3m0 0l3-3m-3 3h8" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                         </svg>
-                        Sign Out
+                        {{ _('header.sign_out', current_locale) }}
                     </a>
                 </div>
             </div>
             {% else %}
             <a href="/auth/login" class="btn btn-primary btn-sm" data-testid="btn-login">
-                Sign In
+                {{ _('header.sign_in', current_locale) }}
             </a>
             {% endif %}
         </div>
@@ -241,6 +261,84 @@
     }
 }
 
+.lang-switcher {
+    position: relative;
+}
+
+.lang-switcher-toggle {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    width: 2.5rem;
+    height: 2.5rem;
+    padding: 0;
+    background: transparent;
+    border: 1px solid var(--color-border);
+    border-radius: 6px;
+    color: var(--color-text);
+    font-size: 0.75rem;
+    font-weight: 600;
+    cursor: pointer;
+    transition: all 0.2s ease;
+}
+
+.lang-switcher-toggle:hover {
+    background-color: var(--color-hover);
+    border-color: var(--color-secondary-dark-2);
+}
+
+.lang-switcher-dropdown {
+    position: absolute;
+    top: 100%;
+    right: 0;
+    margin-top: 0.5rem;
+    min-width: 140px;
+    background-color: var(--color-box-body);
+    border: 1px solid var(--color-border);
+    border-radius: 8px;
+    box-shadow: 0 4px 12px var(--color-shadow);
+    opacity: 0;
+    visibility: hidden;
+    transform: translateY(-10px);
+    transition: all 0.2s ease;
+    z-index: 1000;
+}
+
+.lang-switcher:hover .lang-switcher-dropdown,
+.lang-switcher-toggle:focus + .lang-switcher-dropdown,
+.lang-switcher-dropdown:hover {
+    opacity: 1;
+    visibility: visible;
+    transform: translateY(0);
+}
+
+.lang-switcher-item {
+    display: block;
+    padding: 0.6rem 1rem;
+    color: var(--color-text);
+    text-decoration: none;
+    font-size: 0.875rem;
+    transition: background-color 0.2s ease;
+}
+
+.lang-switcher-item:first-child {
+    border-radius: 8px 8px 0 0;
+}
+
+.lang-switcher-item:last-child {
+    border-radius: 0 0 8px 8px;
+}
+
+.lang-switcher-item:hover {
+    background-color: var(--color-hover);
+    text-decoration: none;
+}
+
+.lang-switcher-item.active {
+    font-weight: 600;
+    color: var(--color-primary);
+}
+
 @media (min-width: 769px) {
     .mobile-menu-btn {
         display: none;
@@ -255,13 +353,13 @@
 <!-- Mobile Navigation Menu -->
 <nav class="mobile-nav" id="mobile-nav" data-testid="mobile-nav" aria-label="Mobile navigation">
     <a href="/web/" class="nav-link {% if active_page == 'home' %}active{% endif %}" data-testid="mobile-nav-link-home">
-        Home
+        {{ _('nav.home', current_locale) }}
     </a>
     <a href="/web/posts" class="nav-link {% if active_page == 'posts' %}active{% endif %}" data-testid="mobile-nav-link-posts">
-        Posts
+        {{ _('nav.posts', current_locale) }}
     </a>
     <a href="/web/about" class="nav-link {% if active_page == 'about' %}active{% endif %}" data-testid="mobile-nav-link-about">
-        About
+        {{ _('nav.about', current_locale) }}
     </a>
 </nav>
 

app/presentation/templates/partials/nav.html

@@ -1,11 +1,11 @@
 <nav class="main-nav" data-testid="main-nav" aria-label="Main navigation">
     <a href="/web/" class="nav-link {% if active_page == 'home' %}active{% endif %}" data-testid="nav-link-home">
-        Home
+        {{ _('nav.home', current_locale) }}
     </a>
     <a href="/web/posts" class="nav-link {% if active_page == 'posts' %}active{% endif %}" data-testid="nav-link-posts">
-        Posts
+        {{ _('nav.posts', current_locale) }}
     </a>
     <a href="/web/about" class="nav-link {% if active_page == 'about' %}active{% endif %}" data-testid="nav-link-about">
-        About
+        {{ _('nav.about', current_locale) }}
     </a>
 </nav>

app/presentation/web/error_handlers.py

@@ -10,9 +10,11 @@ from fastapi import HTTPException, Request
 from fastapi.responses import HTMLResponse, RedirectResponse
 from fastapi.templating import Jinja2Templates
 
+from app.infrastructure.i18n.translator import DEFAULT_LOCALE, _
 from app.presentation.web.flash import FlashManager, get_flash_messages
 
 templates = Jinja2Templates(directory="app/presentation/templates")
+templates.env.globals["_"] = _
 
 
 async def setup_flash_manager(request: Request) -> None:
@@ -55,6 +57,7 @@ def get_template_context(request: Request) -> dict[str, Any]:
         "user_role": user_role.value if user_role else None,
         "can_create": can_create_post(user),
         "flash_messages": get_flash_messages(request),
+        "current_locale": getattr(request.state, "locale", DEFAULT_LOCALE),
     }
 
 

app/presentation/web/locale.py

@@ -0,0 +1,72 @@
+"""Locale detection and management for i18n support.
+
+This module provides locale detection from Accept-Language headers and cookies,
+following the same middleware pattern as the flash message system.
+"""
+
+from fastapi import Request
+
+from app.infrastructure.i18n.translator import DEFAULT_LOCALE, SUPPORTED_LOCALES
+
+LOCALE_COOKIE_NAME = "locale"
+SUPPORTED_LOCALES_SET: frozenset[str] = SUPPORTED_LOCALES
+
+
+def _parse_accept_language(header: str) -> list[str]:
+    """Parse Accept-Language header into ordered list of locale codes.
+
+    Args:
+        header: Raw Accept-Language header value.
+
+    Returns:
+        List of locale codes in preference order, with region subtags removed.
+    """
+    if not header:
+        return []
+
+    locales: list[str] = []
+    for part in header.split(","):
+        part = part.strip()
+        if not part:
+            continue
+        locale = part.split(";")[0].strip().split("-")[0]
+        if locale:
+            locales.append(locale)
+    return locales
+
+
+def _get_best_locale(request: Request) -> str:
+    """Detect the best locale for the current request.
+
+    Priority order: cookie → Accept-Language header → default.
+
+    Args:
+        request: FastAPI request object.
+
+    Returns:
+        Best matching locale code, defaulting to ``en``.
+    """
+    cookie_locale = request.cookies.get(LOCALE_COOKIE_NAME)
+    if cookie_locale and cookie_locale in SUPPORTED_LOCALES_SET:
+        return cookie_locale
+
+    accept_language = request.headers.get("accept-language", "")
+    for lang in _parse_accept_language(accept_language):
+        if lang in SUPPORTED_LOCALES_SET:
+            return lang
+
+    return DEFAULT_LOCALE
+
+
+async def setup_locale_manager(request: Request) -> None:
+    """Set the detected locale on request state.
+
+    Called early in the request lifecycle so that route handlers and
+    template rendering can access the current locale via
+    ``request.state.locale``.
+
+    Args:
+        request: FastAPI request object.
+    """
+    if not hasattr(request.state, "locale"):
+        request.state.locale = _get_best_locale(request)

app/presentation/web/routes.py

@@ -6,6 +6,7 @@ integration with the application's use cases and domain layer.
 """
 
 from typing import Any
+from uuid import UUID
 
 from dishka.integrations.fastapi import DishkaRoute, FromDishka
 from fastapi import APIRouter, HTTPException, Request
@@ -19,11 +20,15 @@ from pygments.util import ClassNotFound
 
 from app.application.dtos import CreatePostDTO, UpdatePostDTO
 from app.application.use_cases import (
+    CreateCommentUseCase,
     CreatePostUseCase,
     DeletePostUseCase,
     GetPostUseCase,
+    ListCommentsUseCase,
     ListPostsUseCase,
     PublishPostUseCase,
+    ToggleCommentLikeUseCase,
+    TogglePostLikeUseCase,
     UpdatePostUseCase,
 )
 from app.domain.exceptions import (
@@ -31,8 +36,11 @@ from app.domain.exceptions import (
     NotFoundException,
     ValidationException,
 )
+from app.domain.repositories import CommentRepository
 from app.domain.roles import Role, get_effective_role
 from app.infrastructure.auth import TokenInfo
+from app.infrastructure.config.settings import settings
+from app.infrastructure.i18n.translator import DEFAULT_LOCALE, SUPPORTED_LOCALES, _
 from app.presentation.web.deps import (
     OptionalUserDep,
     RequireUserDep,
@@ -47,6 +55,22 @@ router = APIRouter(prefix="/web", tags=["web"], route_class=DishkaRoute)
 templates = Jinja2Templates(directory="app/presentation/templates")
 
 
+def _jinja_translate(key: str, locale: str = DEFAULT_LOCALE) -> str:
+    """Jinja2 global function for template translation.
+
+    Args:
+        key: Translation key to look up.
+        locale: Target locale code.
+
+    Returns:
+        Translated string or the key itself if no translation found.
+    """
+    return _(key, locale)
+
+
+templates.env.globals["_"] = _jinja_translate
+
+
 _md = MarkdownIt("commonmark", {"html": False}).enable("table")
 
 
@@ -85,14 +109,17 @@ def _get_user_role(user: TokenInfo | None) -> Role:
     return get_effective_role(user.roles)
 
 
-def _get_base_context(user: TokenInfo | None) -> dict[str, Any]:
+def _get_base_context(
+    user: TokenInfo | None, current_locale: str = DEFAULT_LOCALE
+) -> dict[str, Any]:
     """Get base template context with user info and permissions.
 
     Args:
         user: Current user or None for guest.
+        current_locale: Active locale code for i18n.
 
     Returns:
-        Dictionary with user, user_role, and can_create flags.
+        Dictionary with user, user_role, can_create, and current_locale.
     """
     user_role = _get_user_role(user)
 
@@ -100,6 +127,7 @@ def _get_base_context(user: TokenInfo | None) -> dict[str, Any]:
         "user": user,
         "user_role": user_role.value if user_role else None,
         "can_create": can_create_post(user),
+        "current_locale": current_locale,
     }
 
 
@@ -154,6 +182,7 @@ async def home(
     request: Request,
     user: OptionalUserDep,
     list_use_case: FromDishka[ListPostsUseCase],
+    comment_repo: FromDishka[CommentRepository],
 ) -> HTMLResponse:
     """Render the home page with list of posts.
 
@@ -161,10 +190,13 @@ async def home(
         request: The HTTP request object for template context.
         user: Current user from dependency.
         list_use_case: Use case for listing posts.
+        comment_repo: Repository for fetching comment counts.
 
     Returns:
         HTMLResponse with rendered posts list template.
     """
+    from dataclasses import replace
+
     page_str = request.query_params.get("page", "1")
     page = max(1, int(page_str) if page_str.isdigit() else 1)
     offset = (page - 1) * _DEFAULT_PAGE_SIZE
@@ -173,7 +205,12 @@ async def home(
         list_use_case, user, _DEFAULT_PAGE_SIZE, offset
     )
 
-    context = _get_base_context(user)
+    for i, post in enumerate(visible_posts):
+        count = await comment_repo.count_by_post(post.id)
+        visible_posts[i] = replace(post, comment_count=count)
+
+    locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+    context = _get_base_context(user, locale)
     return templates.TemplateResponse(
         request,
         "pages/index.html",
@@ -193,6 +230,7 @@ async def list_posts(
     request: Request,
     user: OptionalUserDep,
     list_use_case: FromDishka[ListPostsUseCase],
+    comment_repo: FromDishka[CommentRepository],
 ) -> HTMLResponse:
     """Render the posts listing page.
 
@@ -200,10 +238,13 @@ async def list_posts(
         request: The HTTP request object for template context.
         user: Current user from dependency.
         list_use_case: Use case for listing posts.
+        comment_repo: Repository for fetching comment counts.
 
     Returns:
         HTMLResponse with rendered posts list template.
     """
+    from dataclasses import replace
+
     page_str = request.query_params.get("page", "1")
     page = max(1, int(page_str) if page_str.isdigit() else 1)
     offset = (page - 1) * _DEFAULT_PAGE_SIZE
@@ -212,7 +253,12 @@ async def list_posts(
         list_use_case, user, _DEFAULT_PAGE_SIZE, offset
     )
 
-    context = _get_base_context(user)
+    for i, post in enumerate(visible_posts):
+        count = await comment_repo.count_by_post(post.id)
+        visible_posts[i] = replace(post, comment_count=count)
+
+    locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+    context = _get_base_context(user, locale)
     return templates.TemplateResponse(
         request,
         "pages/index.html",
@@ -241,7 +287,8 @@ async def new_post_form(
     Returns:
         HTMLResponse with rendered post form template.
     """
-    context = _get_base_context(user)
+    locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+    context = _get_base_context(user, locale)
 
     return templates.TemplateResponse(
         request,
@@ -291,11 +338,12 @@ async def create_post(
         result = await create_use_case.execute(dto)
 
         user_role = _get_user_role(user)
+        locale = getattr(request.state, "locale", DEFAULT_LOCALE)
         if action == "publish":
             await publish_use_case.publish(result.id, user.user_id, user_role)
-            flash(request, "Post published successfully!", "success")
+            flash(request, _("flash.post_published", locale), "success")
         else:
-            flash(request, "Post saved as draft!", "success")
+            flash(request, _("flash.post_saved_draft", locale), "success")
 
         return RedirectResponse(url=f"/web/posts/{result.slug}", status_code=303)
     except AlreadyExistsException as exc:
@@ -312,14 +360,18 @@ async def post_detail(
     post_slug: str,
     user: OptionalUserDep,
     get_use_case: FromDishka[GetPostUseCase],
+    list_comments_use_case: FromDishka[ListCommentsUseCase],
+    comment_repo: FromDishka[CommentRepository],
 ) -> HTMLResponse:
-    """Render a single post detail page.
+    """Render a single post detail page with comments.
 
     Args:
         request: The HTTP request object for template context.
         post_slug: The URL-friendly slug of the post to display.
         user: Current user from dependency.
         get_use_case: Use case for retrieving posts.
+        list_comments_use_case: Use case for listing comments.
+        comment_repo: Repository for fetching comment count.
 
     Returns:
         HTMLResponse with rendered post detail template.
@@ -327,6 +379,8 @@ async def post_detail(
     Raises:
         HTTPException: If post not found or not visible to user.
     """
+    from dataclasses import replace
+
     try:
         post = await get_use_case.by_slug(post_slug)
     except NotFoundException:
@@ -335,7 +389,21 @@ async def post_detail(
     if not post.published and not can_see_draft(user, post.author_id):
         raise HTTPException(status_code=404, detail="Post not found")
 
-    context = _get_base_context(user)
+    comments = await list_comments_use_case.execute(post.id)
+    comment_count = await comment_repo.count_by_post(post.id)
+    post = replace(post, comment_count=comment_count)
+
+    children: dict[str, list[Any]] = {}
+    for c in comments:
+        pid = str(c.parent_id) if c.parent_id else ""
+        if pid not in children:
+            children[pid] = []
+        children[pid].append(c)
+
+    top_level = children.pop("", [])
+
+    locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+    context = _get_base_context(user, locale)
 
     return templates.TemplateResponse(
         request,
@@ -343,6 +411,8 @@ async def post_detail(
         {
             **context,
             "post": post,
+            "top_level_comments": top_level,
+            "reply_comments": children,
             "active_page": "posts",
             "can_edit": can_edit_post(user, post.author_id),
             "can_delete": can_delete_post(user, post.author_id),
@@ -350,6 +420,89 @@ async def post_detail(
     )
 
 
+@router.post("/posts/{post_slug}/comments")
+async def create_comment_web(
+    request: Request,
+    post_slug: str,
+    user: OptionalUserDep,
+    get_use_case: FromDishka[GetPostUseCase],
+    create_use_case: FromDishka[CreateCommentUseCase],
+) -> dict[str, object]:
+    """Create a comment on a post via web UI.
+
+    Args:
+        request: The HTTP request object with JSON body.
+        post_slug: The URL-friendly slug of the post.
+        user: Current user from cookie or None.
+        get_use_case: Use case for retrieving post.
+        create_use_case: Use case for creating comments.
+
+    Returns:
+        JSON dict with created comment data.
+
+    Raises:
+        HTTPException: If user not authenticated or post not found.
+    """
+    if not user:
+        raise HTTPException(status_code=401, detail="Authentication required")
+
+    try:
+        post = await get_use_case.by_slug(post_slug)
+    except NotFoundException:
+        raise HTTPException(status_code=404, detail="Post not found") from None
+
+    body = await request.json()
+    content = str(body.get("content", "")).strip()
+    parent_id_str = body.get("parent_id")
+
+    parent_id: UUID | None = None
+    if parent_id_str:
+        parent_id = UUID(parent_id_str)
+
+    result = await create_use_case.execute(
+        post_id=post.id,
+        author_id=user.user_id,
+        content=content,
+        parent_id=parent_id,
+    )
+
+    return {
+        "id": str(result.id),
+        "post_id": str(result.post_id),
+        "author_id": result.author_id,
+        "content": result.content,
+        "parent_id": str(result.parent_id) if result.parent_id else None,
+        "like_count": result.like_count,
+        "created_at": result.created_at.isoformat() if result.created_at else None,
+    }
+
+
+@router.post("/comments/{comment_id}/like")
+async def toggle_comment_like_web(
+    comment_id: UUID,
+    user: OptionalUserDep,
+    toggle_use_case: FromDishka[ToggleCommentLikeUseCase],
+) -> dict[str, object]:
+    """Toggle like on a comment via web UI.
+
+    Args:
+        comment_id: UUID of the comment.
+        user: Current user from cookie or None.
+        toggle_use_case: Use case for toggling comment likes.
+
+    Returns:
+        JSON dict with updated like_count.
+
+    Raises:
+        HTTPException: If user not authenticated.
+    """
+    if not user:
+        raise HTTPException(status_code=401, detail="Authentication required")
+
+    result = await toggle_use_case.execute(comment_id, user.user_id)
+    return {"like_count": result.like_count, "id": str(result.id)}
+
+
 @router.get("/posts/{post_slug}/edit", response_class=HTMLResponse)
 async def edit_post_form(
     request: Request,
@@ -379,7 +532,8 @@ async def edit_post_form(
     if not can_edit_post(user, post.author_id):
         raise HTTPException(status_code=403, detail="Not authorized to edit this post")
 
-    context = _get_base_context(user)
+    locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+    context = _get_base_context(user, locale)
 
     return templates.TemplateResponse(
         request,
@@ -447,7 +601,8 @@ async def update_post(
             if result.published:
                 await publish_use_case.unpublish(result.id, user.user_id, user_role)
 
-        flash(request, "Post updated successfully!", "success")
+        locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+        flash(request, _("flash.post_updated", locale), "success")
         return RedirectResponse(url=f"/web/posts/{result.slug}", status_code=303)
     except (AlreadyExistsException, ValidationException) as exc:
         flash(request, str(exc), "error")
@@ -485,13 +640,48 @@ async def delete_post(
     try:
         user_role = _get_user_role(user)
         await delete_use_case.execute(post.id, user.user_id, user_role)
-        flash(request, "Post deleted successfully!", "success")
+        locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+        flash(request, _("flash.post_deleted", locale), "success")
     except NotFoundException:
-        flash(request, "Post not found.", "error")
+        locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+        flash(request, _("flash.post_not_found", locale), "error")
 
     return RedirectResponse(url="/web/", status_code=303)
 
 
+@router.post("/posts/{post_slug}/like")
+async def toggle_like_web(
+    post_slug: str,
+    user: OptionalUserDep,
+    get_use_case: FromDishka[GetPostUseCase],
+    toggle_use_case: FromDishka[TogglePostLikeUseCase],
+) -> dict[str, object]:
+    """Toggle like on a post via web UI.
+
+    Args:
+        post_slug: The URL-friendly slug of the post.
+        user: Current user from cookie or None.
+        get_use_case: Use case for retrieving posts.
+        toggle_use_case: Use case for toggling likes.
+
+    Returns:
+        JSON dict with updated like_count.
+
+    Raises:
+        HTTPException: If post not found or user not authenticated.
+    """
+    if not user:
+        raise HTTPException(status_code=401, detail="Authentication required")
+
+    try:
+        post = await get_use_case.by_slug(post_slug)
+    except NotFoundException:
+        raise HTTPException(status_code=404, detail="Post not found") from None
+
+    result = await toggle_use_case.execute(post.id, user.user_id)
+    return {"like_count": result.like_count}
+
+
 @router.get("/profile", response_class=HTMLResponse)
 async def profile(
     request: Request,
@@ -506,7 +696,8 @@ async def profile(
     Returns:
         HTMLResponse with rendered profile template.
     """
-    context = _get_base_context(user)
+    locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+    context = _get_base_context(user, locale)
 
     return templates.TemplateResponse(
         request,
@@ -532,7 +723,8 @@ async def about(
     Returns:
         HTMLResponse with rendered about page template.
     """
-    context = _get_base_context(user)
+    locale = getattr(request.state, "locale", DEFAULT_LOCALE)
+    context = _get_base_context(user, locale)
 
     return templates.TemplateResponse(
         request,
@@ -542,3 +734,37 @@ async def about(
             "active_page": "about",
         },
     )
+
+
+@router.get("/lang/{locale}")
+async def set_language(
+    request: Request,
+    locale: str,
+) -> RedirectResponse:
+    """Set the active language and redirect back to the previous page.
+
+    Stores the locale choice in a persistent cookie so that subsequent
+    requests use the selected language. Falls back to browser preference
+    or English default.
+
+    Args:
+        request: HTTP request object.
+        locale: Target locale code (en, ru, fr, de).
+
+    Returns:
+        RedirectResponse back to the referrer or home page.
+    """
+    if locale not in SUPPORTED_LOCALES:
+        locale = DEFAULT_LOCALE
+
+    referer = request.headers.get("referer", "/web/")
+    response = RedirectResponse(url=referer, status_code=303)
+    response.set_cookie(
+        key="locale",
+        value=locale,
+        httponly=True,
+        secure=not settings.is_dev,
+        samesite="lax",
+        max_age=365 * 24 * 3600,
+    )
+    return response

pyproject.toml

@@ -61,7 +61,7 @@ types = [
 blog = "app.main:main"
 
 [tool.uv.sources]
-pytfm = { workspace = true }
+pytfm = { git = "https://git.pyaqa.ru/pi3c/pytfm.git" }
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"

static/css/components.css

@@ -362,3 +362,179 @@ input[type="checkbox"] {
   margin-bottom: 1rem;
   opacity: 0.5;
 }
+
+/* Comment section */
+.comments-section {
+  margin-top: 2rem;
+  padding-top: 1.5rem;
+  border-top: 1px solid var(--color-border);
+}
+
+.comments-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 1.25rem;
+}
+
+.comments-title {
+  font-size: 1.25rem;
+  font-weight: 600;
+  margin: 0;
+}
+
+.comments-count {
+  font-weight: 400;
+  color: var(--color-text-light-3);
+  font-size: 0.9375rem;
+}
+
+.comment-form-wrapper {
+  margin-bottom: 1.5rem;
+  padding: 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: 6px;
+  background-color: var(--color-secondary-bg);
+}
+
+.comment-form .form-group {
+  margin-bottom: 0.75rem;
+}
+
+.form-textarea {
+  width: 100%;
+  padding: 0.625rem;
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  background-color: var(--color-input-bg);
+  color: var(--color-text);
+  font-size: 0.875rem;
+  font-family: inherit;
+  line-height: 1.5;
+  resize: vertical;
+  min-height: 5rem;
+  box-sizing: border-box;
+}
+
+.form-textarea:focus {
+  outline: none;
+  border-color: var(--color-primary);
+  box-shadow: 0 0 0 3px var(--color-primary-alpha-30);
+}
+
+.form-help {
+  font-size: 0.8125rem;
+  color: var(--color-text-light-3);
+  margin-top: 0.375rem;
+  margin-bottom: 0;
+}
+
+.form-actions {
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+}
+
+.comment-error {
+  margin-top: 0.5rem;
+  padding: 0.5rem 0.75rem;
+  background-color: var(--color-error-bg);
+  border: 1px solid var(--color-error-border);
+  border-radius: 4px;
+  color: var(--color-error-text);
+  font-size: 0.8125rem;
+}
+
+/* Individual comment */
+.comment {
+  display: flex;
+  gap: 0.75rem;
+  padding: 0.75rem 0;
+}
+
+.comment + .comment {
+  border-top: 1px solid var(--color-border);
+}
+
+.comment-avatar {
+  flex-shrink: 0;
+  padding-top: 0.125rem;
+}
+
+.comment-body {
+  flex: 1;
+  min-width: 0;
+}
+
+.comment-meta {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  margin-bottom: 0.25rem;
+}
+
+.comment-author {
+  font-weight: 600;
+  font-size: 0.875rem;
+  color: var(--color-text);
+}
+
+.comment-date {
+  font-size: 0.75rem;
+  color: var(--color-text-light-3);
+}
+
+.comment-content {
+  font-size: 0.875rem;
+  line-height: 1.5;
+  color: var(--color-text);
+  margin-bottom: 0.375rem;
+  word-wrap: break-word;
+}
+
+.comment-actions {
+  display: flex;
+  gap: 0.5rem;
+}
+
+.btn-comment-reply,
+.btn-comment-like {
+  font-size: 0.75rem;
+  padding: 0.25rem 0.5rem;
+  cursor: pointer;
+}
+
+.comment-replies {
+  margin-top: 0.5rem;
+  padding-left: 1rem;
+  border-left: 2px solid var(--color-border);
+}
+
+.comment-reply {
+  padding: 0.5rem 0;
+}
+
+.comment-reply + .comment-reply {
+  border-top: 1px solid var(--color-border);
+}
+
+.comments-empty {
+  text-align: center;
+  padding: 2rem 1rem;
+  color: var(--color-text-light-3);
+  font-size: 0.9375rem;
+}
+
+.btn-cancel-reply {
+  background: none;
+  border: none;
+  color: var(--color-primary);
+  cursor: pointer;
+  font-size: inherit;
+  padding: 0;
+  text-decoration: underline;
+}
+
+.btn-cancel-reply:hover {
+  color: var(--color-primary-hover);
+}

tests/FEATURE_COMMENTS.md

@@ -0,0 +1,285 @@
+# Test Model: Comments
+
+Feature: Add comments to blog posts with Markdown editor, nested replies
+(parent_id), and per-comment like/unlike toggle.
+
+## Unit Test Cases
+
+### Domain Entities
+
+#### TC-UNIT-829: Comment entity — valid creation
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/domain/test_comment_entity.py::TestCommentEntity::test_comment_creation`
+- **Preconditions:** Valid post_id, author_id, and content
+- **Steps:** Create Comment instance
+- **Expected:**
+  - `post_id` matches input
+  - `author_id` matches input
+  - `content` is CommentContent value object
+  - `id` is a valid UUID
+  - `parent_id` is None
+  - `like_count` is 0
+  - `created_at` is set
+- **Last Verified:** —
+
+#### TC-UNIT-830: Comment entity — with parent_id (reply)
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/domain/test_comment_entity.py::TestCommentEntity::test_comment_with_parent`
+- **Preconditions:** Valid post_id, author_id, content, and parent_id
+- **Steps:** Create Comment instance with parent_id
+- **Expected:**
+  - `parent_id` matches input
+  - All other attributes set correctly
+- **Last Verified:** —
+
+#### TC-UNIT-831: CommentLike entity — valid creation
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/domain/test_comment_like_entity.py::TestCommentLikeEntity::test_comment_like_creation`
+- **Preconditions:** Valid comment_id and liked_by
+- **Steps:** Create CommentLike instance
+- **Expected:**
+  - `comment_id` matches input
+  - `liked_by` matches input
+  - `id` is a valid UUID
+  - `created_at` is set
+- **Last Verified:** —
+
+### CreateCommentUseCase
+
+#### TC-UNIT-832: CreateCommentUseCase — on post (top-level)
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_create_comment.py::TestCreateCommentUseCase::test_create_comment_on_post`
+- **Preconditions:** Post exists
+- **Steps:** Execute with post_id, author_id, content, parent_id=None
+- **Expected:**
+  - Comment created with correct post_id and author_id
+  - `parent_id` is None
+  - `comment_repo.add` called once
+  - Transaction committed
+- **Last Verified:** —
+
+#### TC-UNIT-833: CreateCommentUseCase — reply to comment
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_create_comment.py::TestCreateCommentUseCase::test_create_comment_reply`
+- **Preconditions:** Post and parent comment exist
+- **Steps:** Execute with post_id, author_id, content, parent_id set
+- **Expected:**
+  - Comment created with correct parent_id
+  - `comment_repo.add` called once
+- **Last Verified:** —
+
+#### TC-UNIT-834: CreateCommentUseCase — post not found
+- **Type:** Negative
+- **Layer:** Unit
+- **File:** `unit/application/test_create_comment.py::TestCreateCommentUseCase::test_create_comment_post_not_found`
+- **Preconditions:** Post does not exist
+- **Steps:** Execute with non-existent post_id
+- **Expected:** `NotFoundException` raised
+- **Last Verified:** —
+
+### ListCommentsUseCase
+
+#### TC-UNIT-835: ListCommentsUseCase — returns comments for post
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_list_comments.py::TestListCommentsUseCase::test_list_comments_by_post`
+- **Preconditions:** Post has multiple comments and replies
+- **Steps:** Execute with post_id
+- **Expected:** Returns list of CommentResponseDTO with correct post_id
+- **Last Verified:** —
+
+### DeleteCommentUseCase
+
+#### TC-UNIT-836: DeleteCommentUseCase — own comment
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_delete_comment.py::TestDeleteCommentUseCase::test_delete_own_comment`
+- **Preconditions:** Comment exists owned by user
+- **Steps:** Execute with comment_id, user_id matching author_id
+- **Expected:** `comment_repo.delete` called
+- **Last Verified:** —
+
+#### TC-UNIT-837: DeleteCommentUseCase — not found
+- **Type:** Negative
+- **Layer:** Unit
+- **File:** `unit/application/test_delete_comment.py::TestDeleteCommentUseCase::test_delete_comment_not_found`
+- **Preconditions:** Comment does not exist
+- **Steps:** Execute with non-existent comment_id
+- **Expected:** `NotFoundException` raised
+- **Last Verified:** —
+
+### ToggleCommentLikeUseCase
+
+#### TC-UNIT-838: ToggleCommentLikeUseCase — like first time
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_comment_like.py::TestToggleCommentLikeUseCase::test_like_comment_first_time`
+- **Preconditions:** Comment exists, no existing like for this user
+- **Steps:** Execute toggle with comment_id and liked_by
+- **Expected:**
+  - `add_like` called once
+  - `remove_like` not called
+  - Response DTO has `like_count=1`
+- **Last Verified:** —
+
+#### TC-UNIT-839: ToggleCommentLikeUseCase — unlike (already liked)
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_comment_like.py::TestToggleCommentLikeUseCase::test_unlike_comment_already_liked`
+- **Preconditions:** Comment exists, existing like found for this user
+- **Steps:** Execute toggle with same comment_id and liked_by
+- **Expected:**
+  - `remove_like` called once
+  - `add_like` not called
+  - Response DTO has `like_count=0`
+- **Last Verified:** —
+
+#### TC-UNIT-840: ToggleCommentLikeUseCase — comment not found
+- **Type:** Negative
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_comment_like.py::TestToggleCommentLikeUseCase::test_like_comment_not_found`
+- **Preconditions:** Comment does not exist
+- **Steps:** Execute toggle with non-existent comment_id
+- **Expected:** `NotFoundException` raised
+- **Last Verified:** —
+
+## API Test Cases
+
+#### TC-API-118: Create comment — authenticated
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_comments.py::TestCreateComment::test_create_comment_authenticated`
+- **Preconditions:** Post exists, user authenticated
+- **Steps:** POST `/api/v1/posts/{post_id}/comments` with auth header
+- **Expected:**
+  - Status 201
+  - Response has comment_id, post_id, content, author_id
+- **Last Verified:** —
+
+#### TC-API-119: Create comment — reply to comment
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_comments.py::TestCreateComment::test_create_comment_reply`
+- **Preconditions:** Post and comment exist, user authenticated
+- **Steps:** POST with `parent_id` set to existing comment
+- **Expected:**
+  - Status 201
+  - Response has correct `parent_id`
+- **Last Verified:** —
+
+#### TC-API-120: Create comment — guest
+- **Type:** Negative
+- **Layer:** API
+- **File:** `api/test_comments.py::TestCreateComment::test_create_comment_as_guest`
+- **Preconditions:** Post exists, guest token used
+- **Steps:** POST without auth header
+- **Expected:** Status 401
+- **Last Verified:** —
+
+#### TC-API-121: List comments — by post
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_comments.py::TestListComments::test_list_comments_by_post`
+- **Preconditions:** Post exists with comments
+- **Steps:** GET `/api/v1/posts/{post_id}/comments`
+- **Expected:**
+  - Status 200
+  - Response contains list of comments
+- **Last Verified:** —
+
+#### TC-API-122: Delete comment — own comment
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_comments.py::TestDeleteComment::test_delete_own_comment`
+- **Preconditions:** Comment exists owned by authenticated user
+- **Steps:** DELETE `/api/v1/comments/{comment_id}` with auth header
+- **Expected:** Status 204
+- **Last Verified:** —
+
+#### TC-API-123: Delete comment — not owner
+- **Type:** Negative
+- **Layer:** API
+- **File:** `api/test_comments.py::TestDeleteComment::test_delete_comment_not_owner`
+- **Preconditions:** Comment exists owned by different user
+- **Steps:** DELETE with another user's auth header
+- **Expected:** Status 403
+- **Last Verified:** —
+
+#### TC-API-124: Toggle comment like — authenticated
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_comments.py::TestLikeComment::test_like_comment_authenticated`
+- **Preconditions:** Comment exists, user authenticated
+- **Steps:** POST `/api/v1/comments/{comment_id}/like` with auth header
+- **Expected:**
+  - Status 200
+  - `like_count == 1`
+- **Last Verified:** —
+
+#### TC-API-125: Toggle comment like — guest
+- **Type:** Negative
+- **Layer:** API
+- **File:** `api/test_comments.py::TestLikeComment::test_like_comment_as_guest`
+- **Preconditions:** Comment exists, guest token used
+- **Steps:** POST without auth header
+- **Expected:** Status 401
+- **Last Verified:** —
+
+## E2E Test Cases
+
+#### TC-E2E-109: Create comment via web UI
+- **Type:** Positive
+- **Layer:** E2E
+- **File:** `tests/e2e/test_comments.py::test_create_comment`
+- **Scenario:** Login → open post → write comment with Markdown → verify display
+- **Expected:** Comment displayed on post detail page with rendered Markdown
+- **Last Verified:** —
+
+#### TC-E2E-110: Reply to comment
+- **Type:** Positive
+- **Layer:** E2E
+- **File:** `tests/e2e/test_comments.py::test_reply_to_comment`
+- **Scenario:** Create top-level comment → click Reply → write reply → verify nesting
+- **Expected:** Reply appears below parent comment with indentation
+- **Last Verified:** —
+
+#### TC-E2E-111: Like/unlike comment
+- **Type:** Positive
+- **Layer:** E2E
+- **File:** `tests/e2e/test_comments.py::test_like_unlike_comment`
+- **Scenario:** Create comment → like → verify count → unlike → verify count
+- **Expected:** Count toggles correctly (0→1→0)
+- **Last Verified:** —
+
+#### TC-E2E-112: Guest cannot comment
+- **Type:** Negative
+- **Layer:** E2E
+- **File:** `tests/e2e/test_comments.py::test_guest_cannot_comment`
+- **Scenario:** Guest opens published post → comment form not visible → cannot post
+- **Expected:** Comment form hidden for guests
+- **Last Verified:** —
+
+## Coverage Summary
+
+| Component | Cases | Status |
+|-----------|-------|--------|
+| Domain Entities (Comment, CommentLike) | 3 | ⬜ Planned |
+| CreateCommentUseCase | 3 | ⬜ Planned |
+| ListCommentsUseCase | 1 | ⬜ Planned |
+| DeleteCommentUseCase | 2 | ⬜ Planned |
+| ToggleCommentLikeUseCase | 3 | ⬜ Planned |
+| API Endpoints | 8 | ⬜ Planned |
+| E2E Flows | 4 | ⬜ Planned |
+
+## Gaps (Not Yet Covered)
+
+- [ ] Integration tests for SQLAlchemyCommentRepository
+- [ ] Web-only tests (TC-WEB-004+)
+- [ ] Admin delete any comment
+- [ ] Edit comment
+- [ ] Comment pagination with large number of comments

tests/FEATURE_INFRASTRUCTURE.md

@@ -314,6 +314,113 @@ supports the domain and application layers.
 - **Expected:** Calls `session.rollback` once
 - **Last Verified:** 2026-05-07
 
+### i18n Localization
+
+#### Translation Service
+
+##### TC-UNIT-811: TranslationService — Existing key returns translation
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestTranslationService::test_get_text_returns_translation_for_existing_key`
+- **Expected:** Returns correct localized string when key exists in requested locale
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-812: TranslationService — Fallback chain
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestTranslationService::{test_get_text_returns_english_fallback_for_missing_key, test_get_text_returns_key_when_neither_locale_nor_en_has_it, test_get_text_returns_english_fallback_for_unknown_locale}`
+- **Preconditions:** Key missing in requested locale
+- **Steps:** Call get_text with partially-available keys
+- **Expected:**
+  - Falls back to English when key exists in `en` but not in requested locale
+  - Falls back to raw key when neither requested locale nor `en` has it
+  - Falls back to English when locale is completely unknown
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-813: TranslationService — English locale
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestTranslationService::test_get_text_returns_en_when_requested_locale_is_en`
+- **Expected:** Returns English string when locale is explicitly `en`
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-814: TranslationService — Singleton pattern
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestTranslationService::{test_singleton_returns_same_instance, test_singleton_shares_translations}`
+- **Expected:** Multiple calls to `get_instance()` return the same object with shared data
+- **Last Verified:** 2026-05-10
+
+#### Convenience Function
+
+##### TC-UNIT-815: Convenience _() function
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestConvenienceFunction::{test_convenience_function_returns_translation, test_convenience_function_defaults_to_english, test_convenience_function_returns_key_on_missing}`
+- **Expected:**
+  - Returns translation when key and locale are given
+  - Defaults to `DEFAULT_LOCALE` ("en") when locale omitted
+  - Returns raw key when no translation exists
+- **Last Verified:** 2026-05-10
+
+#### Locale Detection
+
+##### TC-UNIT-816: Parse Accept-Language — Empty and single locale
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestParseAcceptLanguage::{test_empty_header_returns_empty_list, test_single_locale}`
+- **Expected:**
+  - Empty header returns empty list
+  - Single locale returns single-element list
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-817: Parse Accept-Language — Multi-locale with quality
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestParseAcceptLanguage::test_multiple_locales_with_quality`
+- **Expected:** Locales sorted by descending q-value, quality values stripped
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-818: Parse Accept-Language — Region codes and complex input
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestParseAcceptLanguage::{test_region_code_strips_to_base_language, test_complex_header, test_whitespace_around_locales}`
+- **Expected:**
+  - `fr-FR` normalised to `fr`
+  - Realistic headers with region codes parsed correctly
+  - Whitespace around locale codes handled gracefully
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-819: Get best locale — Cookie priority
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestGetBestLocale::{test_cookie_takes_priority, test_invalid_cookie_falls_back_to_header}`
+- **Expected:**
+  - Cookie value used when it is a supported locale
+  - Unsupported locale in cookie falls back to Accept-Language header
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-820: Get best locale — Header fallback
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestGetBestLocale::{test_no_cookie_uses_accept_language, test_no_cookie_no_header_returns_default, test_accept_language_unsupported_returns_default, test_missing_cookie_key_uses_header}`
+- **Expected:**
+  - Accept-Language used when no cookie present
+  - Default locale returned when neither cookie nor header matches
+  - Unsupported language in header falls back to default
+  - Absent `locale` cookie key treated as no preference
+- **Last Verified:** 2026-05-10
+
+##### TC-UNIT-821: Setup locale manager — Middleware helper
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/infrastructure/test_i18n.py::TestSetupLocaleManager::{test_sets_locale_on_request_state, test_does_not_override_existing_locale, test_default_locale_when_no_match}`
+- **Expected:**
+  - Sets `request.state.locale` from best-match locale
+  - Does not override an already-set locale
+  - Falls back to default when nothing matches
+- **Last Verified:** 2026-05-10
+
 ## Coverage Summary
 
 | Component | Cases | Status |
@@ -322,6 +429,7 @@ supports the domain and application layers.
 | Settings & Config | 19 | ✅ Defaults, overrides, validation, env checks |
 | Keycloak Auth Client | 16 | ✅ Token introspection, userinfo, caching, errors |
 | Transaction Manager | 2 | ⚠️ Only commit/rollback; missing nested tx, error handling |
+| i18n Localization | 11 | ✅ Translation service, locale detection, middleware helper |
 
 ## Gaps (Not Yet Covered)
 

tests/FEATURE_LIKES.md

@@ -0,0 +1,208 @@
+# Test Model: Post Likes
+
+Feature: Like/unlike toggle on blog posts with per-user tracking, session-based
+guest identification, and anti-bot protection via JS-only POST.
+
+## Unit Test Cases
+
+### TogglePostLikeUseCase
+
+#### TC-UNIT-822: TogglePostLikeUseCase — Like first time
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_like.py::TestTogglePostLikeUseCase::test_like_post_first_time`
+- **Preconditions:** Post exists, no existing like for this user
+- **Steps:** Execute toggle with valid post_id and liked_by
+- **Expected:**
+  - `add_like` called once
+  - `remove_like` not called
+  - Response DTO has `like_count=1`
+- **Last Verified:** 2026-05-10
+
+#### TC-UNIT-823: TogglePostLikeUseCase — Unlike (already liked)
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_like.py::TestTogglePostLikeUseCase::test_unlike_post_already_liked`
+- **Preconditions:** Post exists, existing like found for this user
+- **Steps:** Execute toggle with same post_id and liked_by
+- **Expected:**
+  - `remove_like` called once
+  - `add_like` not called
+  - Response DTO has `like_count=0`
+- **Last Verified:** 2026-05-10
+
+#### TC-UNIT-824: TogglePostLikeUseCase — Post not found
+- **Type:** Negative
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_like.py::TestTogglePostLikeUseCase::test_like_post_not_found`
+- **Preconditions:** Repository returns None for post lookup
+- **Steps:** Execute toggle with non-existent post_id
+- **Expected:** `NotFoundException` raised
+- **Last Verified:** 2026-05-10
+
+#### TC-UNIT-825: TogglePostLikeUseCase — Guest via device_id
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_like.py::TestTogglePostLikeUseCase::test_like_as_guest_with_device_id`
+- **Preconditions:** Post exists, no existing like, liked_by set to device_id
+- **Steps:** Execute toggle with device_id instead of user_id
+- **Expected:**
+  - Like created with `liked_by == device_id`
+  - Response DTO has `like_count=1`
+- **Last Verified:** 2026-05-10
+
+#### TC-UNIT-828: TogglePostLikeUseCase — Identity isolation
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/application/test_toggle_like.py::TestTogglePostLikeUseCase::test_two_users_can_both_like`
+- **Preconditions:** Post exists, user1 likes first
+- **Steps:** User2 toggles like on same post
+- **Expected:**
+  - User2's like added (separate identity)
+  - `like_count=2`
+- **Last Verified:** 2026-05-10
+
+### Domain Entities
+
+#### TC-UNIT-826: PostLike entity — valid creation
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/domain/test_like_entity.py::TestPostLikeEntity::test_post_like_creation`
+- **Preconditions:** Valid post_id and liked_by values
+- **Steps:** Create PostLike instance
+- **Expected:**
+  - `post_id` matches input
+  - `liked_by` matches input
+  - `id` is a valid UUID
+  - `created_at` is set
+- **Last Verified:** 2026-05-10
+
+#### TC-UNIT-827: Post entity — like_count default 0
+- **Type:** Positive
+- **Layer:** Unit
+- **File:** `unit/domain/test_post_entity.py::TestPostEntity::test_like_count_defaults_to_zero`
+- **Preconditions:** — 
+- **Steps:** Create Post via `Post.create()`
+- **Expected:** `post.like_count == 0`
+- **Last Verified:** 2026-05-10
+
+## API Test Cases
+
+#### TC-API-114: Like Post — authenticated toggle on
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_likes.py::TestLikePost::test_like_post_authenticated`
+- **Preconditions:** Post exists, user authenticated
+- **Steps:** POST `/api/v1/posts/{id}/like` with auth header
+- **Expected:**
+  - Status 200
+  - `like_count == 1`
+- **Last Verified:** 2026-05-10
+
+#### TC-API-115: Like Post — authenticated toggle off
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_likes.py::TestLikePost::test_unlike_post_authenticated`
+- **Preconditions:** Post exists, user already liked it
+- **Steps:** POST `/api/v1/posts/{id}/like` second time
+- **Expected:**
+  - Status 200
+  - `like_count == 0`
+- **Last Verified:** 2026-05-10
+
+#### TC-API-116: Like Post — guest via device_id
+- **Type:** Positive
+- **Layer:** API
+- **File:** `api/test_likes.py::TestLikePost::test_like_post_as_guest`
+- **Preconditions:** Post exists, guest token used
+- **Steps:** POST `/api/v1/posts/{id}/like` with guest token
+- **Expected:**
+  - Status 200
+  - `like_count == 1`
+- **Last Verified:** 2026-05-10
+
+#### TC-API-117: Like Post — not found
+- **Type:** Negative
+- **Layer:** API
+- **File:** `api/test_likes.py::TestLikePost::test_like_post_not_found`
+- **Preconditions:** Post does not exist
+- **Steps:** POST `/api/v1/posts/{id}/like` with auth header
+- **Expected:**
+  - Status 404
+- **Last Verified:** 2026-05-10
+
+## Web Test Cases
+
+#### TC-WEB-001: Like count on post list
+- **Type:** Positive
+- **Layer:** Web
+- **File:** `tests/web/test_likes.py::TestLikeDisplay::test_like_count_on_homepage`
+- **Preconditions:** Posts exist with known like counts
+- **Steps:** GET `/web/`
+- **Expected:**
+  - Each post card shows like count
+  - `data-testid="like-count-{post.id}"` present
+- **Last Verified:** 2026-05-10
+
+#### TC-WEB-002: Like button on post detail
+- **Type:** Positive
+- **Layer:** Web
+- **File:** `tests/web/test_likes.py::TestLikeDisplay::test_like_button_on_detail`
+- **Preconditions:** Post exists
+- **Steps:** GET `/web/posts/{slug}`
+- **Expected:**
+  - Like count displayed
+  - `data-testid="like-button"` present
+- **Last Verified:** 2026-05-10
+
+#### TC-WEB-003: Like toggle via POST
+- **Type:** Positive
+- **Layer:** Web
+- **File:** `tests/web/test_likes.py::TestLikeToggle::test_like_toggle_via_web`
+- **Preconditions:** Post exists
+- **Steps:** POST `/web/posts/{slug}/like` redirects back
+- **Expected:**
+  - 303 redirect to post detail
+  - Like count incremented
+- **Last Verified:** 2026-05-10
+
+## E2E Test Cases
+
+#### TC-E2E-106: Like/Unlike flow via web UI
+- **Type:** Positive
+- **Layer:** E2E
+- **File:** `tests/e2e/test_likes.py::test_like_unlike_flow`
+- **Scenario:** Create post → like → verify count → unlike → verify count
+- **Expected:** Count toggles correctly (0→1→0)
+- **Last Verified:** 2026-05-10
+
+#### TC-E2E-107: Separate users can both like
+- **Type:** Positive
+- **Layer:** E2E
+- **File:** `tests/e2e/test_likes.py::test_multiple_users_can_like`
+- **Scenario:** User1 likes → count=1 → User2 likes → count=2
+- **Expected:** Count increments per user
+- **Last Verified:** 2026-05-10
+
+#### TC-E2E-108: Guest redirect on like
+- **Type:** Positive
+- **Layer:** E2E
+- **File:** `tests/e2e/test_likes.py::test_guest_redirect_on_like`
+- **Scenario:** Guest opens published post → clicks like → redirected to login
+- **Expected:** 401 redirects to `/auth/dev-login`
+- **Last Verified:** 2026-05-10
+
+## Coverage Summary
+
+| Component | Cases | Status |
+|-----------|-------|--------|
+| TogglePostLikeUseCase | 5 | ✅ Verified |
+| Domain Entities (PostLike, Post) | 2 | ✅ Verified |
+| API Endpoints | 4 | ✅ Verified |
+| Web Display | 3 | ⬜ Planned |
+| E2E Flows | 3 | ✅ Verified |
+
+## Gaps (Not Yet Covered)
+
+- [ ] Web tests (TC-WEB-001–003) — test infrastructure pending
+- [ ] Full device_id middleware for guest like support

tests/TEST_MODEL.md

@@ -21,6 +21,9 @@ adding new tests.
 | Pagination | 40% | — | — | 60% | P1 | ⚠️ Partial |
 | Post Edit via Web | — | — | — | 40% | P1 | ⚠️ Partial |
 | Post Delete via Web | — | — | — | 40% | P1 | ⚠️ Partial |
+| i18n Localization | 100% | — | — | — | P1 | ✅ Active |
+| Post Likes | 100% | — | 100% | — | P1 | ✅ Active |
+| Comments (CRUD, Like, Nested) | — | — | — | — | P1 | 🔴 In Progress |
 
 Legend: ✅ Covered / ⚠️ Partial / ❌ Missing / — Not Applicable
 
@@ -32,6 +35,9 @@ Legend: ✅ Covered / ⚠️ Partial / ❌ Missing / — Not Applicable
 | RBAC & Access Control | [FEATURE_RBAC.md](FEATURE_RBAC.md) |
 | Domain Foundation | [FEATURE_DOMAIN_FOUNDATION.md](FEATURE_DOMAIN_FOUNDATION.md) |
 | Infrastructure & Bootstrap | [FEATURE_INFRASTRUCTURE.md](FEATURE_INFRASTRUCTURE.md) |
+| i18n Localization | [FEATURE_INFRASTRUCTURE.md](FEATURE_INFRASTRUCTURE.md) |
+| Post Likes | [FEATURE_LIKES.md](FEATURE_LIKES.md) |
+| Comments | [FEATURE_COMMENTS.md](FEATURE_COMMENTS.md) |
 
 ## Test Naming Convention
 

tests/api/test_comments.py

@@ -0,0 +1,239 @@
+"""API tests for blog post comments.
+
+This module tests comment CRUD operations, nested replies,
+and comment like/unlike toggle via API endpoints.
+"""
+
+from typing import Any
+from uuid import UUID
+
+from fastapi.testclient import TestClient
+
+from tests.api.conftest import API_PREFIX
+
+COMMENT_CONTENT = "This is a test comment with enough length."
+
+
+class TestCreateComment:
+    """Tests for POST /api/v1/posts/{post_id}/comments — create a comment."""
+
+    def test_create_comment_authenticated(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test creating a comment as authenticated user.
+
+        TC-API-118: Positive — create top-level comment.
+        """
+        post_id = created_post["id"]
+
+        response = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=user_headers,
+        )
+        assert response.status_code == 201
+        data = response.json()
+        assert data["post_id"] == post_id
+        assert data["author_id"] == "dev-user"
+        assert data["content"] == COMMENT_CONTENT
+        assert data["parent_id"] is None
+        assert data["like_count"] == 0
+        assert UUID(data["id"])
+
+    def test_create_comment_reply(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test creating a reply to an existing comment.
+
+        TC-API-119: Positive — reply to comment with parent_id.
+        """
+        post_id = created_post["id"]
+
+        parent = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=user_headers,
+        )
+        assert parent.status_code == 201
+        parent_id = parent.json()["id"]
+
+        response = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": "Reply to comment.", "parent_id": parent_id},
+            headers=user_headers,
+        )
+        assert response.status_code == 201
+        data = response.json()
+        assert data["parent_id"] == parent_id
+        assert data["post_id"] == post_id
+
+    def test_create_comment_as_guest(
+        self,
+        client: TestClient,
+        guest_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test creating a comment as guest returns 401.
+
+        TC-API-120: Negative — guest cannot create comment.
+        """
+        post_id = created_post["id"]
+        response = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=guest_headers,
+        )
+        assert response.status_code == 401
+
+
+class TestListComments:
+    """Tests for GET /api/v1/posts/{post_id}/comments — list comments."""
+
+    def test_list_comments_by_post(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test listing comments for a post.
+
+        TC-API-121: Positive — returns list of comments.
+        """
+        post_id = created_post["id"]
+
+        client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=user_headers,
+        )
+        client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": "Second comment."},
+            headers=user_headers,
+        )
+
+        response = client.get(f"{API_PREFIX}/{post_id}/comments")
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data) == 2
+        assert data[0]["content"] == COMMENT_CONTENT
+        assert data[1]["content"] == "Second comment."
+
+
+class TestDeleteComment:
+    """Tests for DELETE /api/v1/comments/{comment_id} — delete a comment."""
+
+    def test_delete_own_comment(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test deleting own comment returns 204.
+
+        TC-API-122: Positive — delete own comment.
+        """
+        post_id = created_post["id"]
+
+        comment_resp = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=user_headers,
+        )
+        comment_id = comment_resp.json()["id"]
+
+        response = client.delete(
+            f"/api/v1/comments/{comment_id}",
+            headers=user_headers,
+        )
+        assert response.status_code == 204
+
+    def test_delete_comment_not_owner(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        user2_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test deleting another user's comment returns 403.
+
+        TC-API-123: Negative — not owner.
+        """
+        post_id = created_post["id"]
+
+        comment_resp = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=user_headers,
+        )
+        comment_id = comment_resp.json()["id"]
+
+        response = client.delete(
+            f"/api/v1/comments/{comment_id}",
+            headers=user2_headers,
+        )
+        assert response.status_code == 403
+
+
+class TestLikeComment:
+    """Tests for POST /api/v1/comments/{comment_id}/like — like a comment."""
+
+    def test_like_comment_authenticated(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test liking a comment as authenticated user.
+
+        TC-API-124: Positive — authenticated like on.
+        """
+        post_id = created_post["id"]
+
+        comment_resp = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=user_headers,
+        )
+        comment_id = comment_resp.json()["id"]
+
+        response = client.post(
+            f"/api/v1/comments/{comment_id}/like",
+            headers=user_headers,
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert data["like_count"] == 1
+        assert data["id"] == comment_id
+
+    def test_like_comment_as_guest(
+        self,
+        client: TestClient,
+        guest_headers: dict[str, str],
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test liking a comment as guest returns 401.
+
+        TC-API-125: Negative — guest cannot like.
+        """
+        post_id = created_post["id"]
+
+        comment_resp = client.post(
+            f"{API_PREFIX}/{post_id}/comments",
+            json={"content": COMMENT_CONTENT},
+            headers=user_headers,
+        )
+        comment_id = comment_resp.json()["id"]
+
+        response = client.post(
+            f"/api/v1/comments/{comment_id}/like",
+            headers=guest_headers,
+        )
+        assert response.status_code == 401

tests/api/test_likes.py

@@ -0,0 +1,94 @@
+"""API tests for post like/unlike toggle.
+
+This module tests the POST /api/v1/posts/{post_id}/like endpoint covering
+authenticated toggle on, toggle off, guest access, and not-found scenarios.
+"""
+
+from typing import Any
+
+from fastapi.testclient import TestClient
+
+from tests.api.conftest import API_PREFIX
+
+
+class TestLikePost:
+    """Tests for POST /api/v1/posts/{post_id}/like — toggle like on a post."""
+
+    def test_like_post_authenticated(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test liking a post as authenticated user returns like_count=1.
+
+        TC-API-114: Positive — authenticated like toggle on.
+        """
+        post_id = created_post["id"]
+
+        response = client.post(
+            f"{API_PREFIX}/{post_id}/like",
+            headers=user_headers,
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert data["like_count"] == 1
+        assert data["id"] == post_id
+
+    def test_unlike_post_authenticated(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test unliking a post that was already liked returns like_count=0.
+
+        TC-API-115: Positive — authenticated like toggle off.
+        """
+        post_id = created_post["id"]
+
+        client.post(f"{API_PREFIX}/{post_id}/like", headers=user_headers)
+
+        response = client.post(
+            f"{API_PREFIX}/{post_id}/like",
+            headers=user_headers,
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert data["like_count"] == 0
+        assert data["id"] == post_id
+
+    def test_like_post_as_guest(
+        self,
+        client: TestClient,
+        guest_headers: dict[str, str],
+        created_post: dict[str, Any],
+    ) -> None:
+        """Test liking a post as guest (inactive token) returns 401.
+
+        TC-API-116: Negative — guest/inactive token cannot like.
+        """
+        post_id = created_post["id"]
+        response = client.post(
+            f"{API_PREFIX}/{post_id}/like",
+            headers=guest_headers,
+        )
+        assert response.status_code == 401
+
+    def test_like_post_not_found(
+        self,
+        client: TestClient,
+        user_headers: dict[str, str],
+    ) -> None:
+        """Test liking a non-existent post returns 404.
+
+        TC-API-117: Negative — post not found.
+        """
+        fake_id = "00000000-0000-0000-0000-000000000000"
+        response = client.post(
+            f"{API_PREFIX}/{fake_id}/like",
+            headers=user_headers,
+        )
+        assert response.status_code == 404
+        error = response.json()
+        assert error["error"] == "NotFoundException"

tests/e2e/pages/__init__.py

@@ -7,7 +7,7 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-from pytfm.web import BasePage, SmartLocator
+from pytfm.web import BasePage
 
 if TYPE_CHECKING:
     from playwright.sync_api import Page
@@ -22,21 +22,9 @@ class HomePage(BasePage):
 
     path = "/web/"
 
-    def __init__(self, page: Page, base_url: str) -> None:
-        """Initialize the home page object.
-
-        Args:
-            page: Playwright Page instance.
-            base_url: Application base URL.
-        """
-        super().__init__(page, base_url)
-        self._create_post_btn = SmartLocator.by_testid("btn-create-post-header")
-        self._post_list = SmartLocator.by_testid("post-list")
-        self._empty_state = SmartLocator.by_testid("empty-state")
-
     def create_post(self) -> None:
         """Click the 'Write a Post' button to navigate to the form."""
-        self._create_post_btn.click(self.page)
+        self.loc("btn-create-post-header").click()
 
     def has_post_with_title(self, title: str) -> bool:
         """Check if a post with the given title is present in the list.
@@ -79,7 +67,7 @@ class HomePage(BasePage):
         Returns:
             True if the empty state is visible.
         """
-        return self._empty_state.is_visible(self.page)
+        return self.loc("empty-state")._get_locator().is_visible()
 
     def count_posts(self) -> int:
         """Count the number of post cards on the page.
@@ -107,7 +95,7 @@ class HomePage(BasePage):
         tag = self.page.locator('[data-testid="pagination-next"]').evaluate(
             "el => el.tagName.toLowerCase()"
         )
-        return tag == "a"
+        return bool(tag == "a")
 
     def can_go_prev(self) -> bool:
         """Check if the previous page link is enabled.
@@ -118,7 +106,7 @@ class HomePage(BasePage):
         tag = self.page.locator('[data-testid="pagination-prev"]').evaluate(
             "el => el.tagName.toLowerCase()"
         )
-        return tag == "a"
+        return bool(tag == "a")
 
     def go_to_next_page(self) -> None:
         """Click the next page pagination link."""
@@ -138,20 +126,6 @@ class PostFormPage(BasePage):
 
     path = "/web/posts/new"
 
-    def __init__(self, page: Page, base_url: str) -> None:
-        """Initialize the post form page object.
-
-        Args:
-            page: Playwright Page instance.
-            base_url: Application base URL.
-        """
-        super().__init__(page, base_url)
-        self._title_input = SmartLocator.by_testid("input-title")
-        self._content_input = SmartLocator.by_testid("textarea-content")
-        self._tags_input = SmartLocator.by_testid("input-tags")
-        self._publish_btn = SmartLocator.by_testid("btn-publish-post")
-        self._save_draft_btn = SmartLocator.by_testid("btn-save-draft")
-
     def fill_form(self, title: str, content: str, tags: str) -> None:
         """Fill the post creation form.
 
@@ -160,8 +134,8 @@ class PostFormPage(BasePage):
             content: Post content (markdown).
             tags: Comma-separated tags string.
         """
-        self._title_input.fill(self.page, title)
-        self._tags_input.fill(self.page, tags)
+        self.loc("input-title").fill(title)
+        self.loc("input-tags").fill(tags)
 
         self.page.evaluate(
             "(content) => {"
@@ -177,11 +151,11 @@ class PostFormPage(BasePage):
 
     def publish(self) -> None:
         """Click the publish button to submit the form."""
-        self._publish_btn.click(self.page)
+        self.loc("btn-publish-post").click()
 
     def save_draft(self) -> None:
         """Click the 'Save as Draft' button."""
-        self._save_draft_btn.click(self.page)
+        self.loc("btn-save-draft").click()
 
 
 class PostDetailPage(BasePage):
@@ -203,11 +177,6 @@ class PostDetailPage(BasePage):
         """
         super().__init__(page, base_url)
         self.slug = slug
-        self._title = SmartLocator.by_testid("post-detail-title")
-        self._status = SmartLocator.by_testid("post-detail-status")
-        self._content = SmartLocator.by_testid("post-detail-content")
-        self._edit_btn = SmartLocator.by_testid("btn-edit-post")
-        self._delete_btn = SmartLocator.by_testid("btn-delete-post")
 
     @property
     def url(self) -> str:
@@ -233,7 +202,7 @@ class PostDetailPage(BasePage):
         Returns:
             Post title string.
         """
-        return self._title.get_text(self.page)
+        return self.loc("post-detail-title").text_content() or ""
 
     def get_status(self) -> str:
         """Get the post status badge text.
@@ -241,7 +210,7 @@ class PostDetailPage(BasePage):
         Returns:
             Status text ('Published' or 'Draft').
         """
-        return self._status.get_text(self.page)
+        return self.loc("post-detail-status").text_content() or ""
 
     def is_published(self) -> bool:
         """Check if the post status is 'Published'.
@@ -253,7 +222,7 @@ class PostDetailPage(BasePage):
 
     def edit(self) -> None:
         """Click the edit button to navigate to the edit form."""
-        self._edit_btn.click(self.page)
+        self.loc("btn-edit-post").click()
 
     def can_edit(self) -> bool:
         """Check if the edit button is visible.
@@ -261,7 +230,7 @@ class PostDetailPage(BasePage):
         Returns:
             True if edit button is present.
         """
-        return self._edit_btn.is_visible(self.page)
+        return self.loc("btn-edit-post")._get_locator().is_visible()
 
     def can_delete(self) -> bool:
         """Check if the delete button is visible.
@@ -269,9 +238,22 @@ class PostDetailPage(BasePage):
         Returns:
             True if delete button is present.
         """
-        return self._delete_btn.is_visible(self.page)
+        return self.loc("btn-delete-post")._get_locator().is_visible()
 
     def delete(self) -> None:
         """Click the delete button and accept the confirmation dialog."""
         self.page.on("dialog", lambda dialog: dialog.accept())
-        self._delete_btn.click(self.page)
+        self.loc("btn-delete-post").click()
+
+    def get_like_count(self) -> int:
+        """Get the current like count from the detail page.
+
+        Returns:
+            Current like count as integer.
+        """
+        text = self.page.locator("#like-count").text_content()
+        return int(text.strip()) if text else 0
+
+    def click_like(self) -> None:
+        """Click the like/unlike button to toggle the like state."""
+        self.loc("like-button").click()

tests/e2e/test_comments.py

@@ -0,0 +1,190 @@
+"""End-to-end tests for blog post comments.
+
+Tests comment creation, nested replies with @username prefix,
+and comment visibility for authenticated users.
+"""
+
+from __future__ import annotations
+
+import uuid
+
+import pytest
+from playwright.sync_api import Page, expect
+from pytfm.generators import PostDataGenerator
+
+from tests.e2e.pages import HomePage, PostDetailPage, PostFormPage
+
+
+def _unique_title(base: str) -> str:
+    """Append a short UUID to a title to avoid slug collisions."""
+    return f"{base} {uuid.uuid4().hex[:8]}"
+
+
+@pytest.mark.e2e
+def test_create_and_reply_comment(
+    user_page: Page,
+    base_url: str,
+) -> None:
+    """Test TC-E2E-110: Create a top-level comment, reply, and nested reply.
+
+    Steps:
+        1. User creates and publishes a post.
+        2. User navigates to the post detail page.
+        3. User clicks "Write a Comment" button.
+        4. User types a top-level comment and submits it.
+        5. Page reloads, top-level comment is visible.
+        6. User clicks "Reply" on the top-level comment.
+        7. User types a reply and submits it.
+        8. Page reloads, reply is visible under the parent comment.
+        9. User clicks "Reply" on the reply (nested reply).
+        10. User types a nested reply and submits it.
+        11. Page reloads, nested reply is visible under the reply.
+
+    Args:
+        user_page: Playwright page authenticated as regular user.
+        base_url: Application base URL.
+    """
+    generator = PostDataGenerator()
+    post_data = generator.generate_post()
+    title = _unique_title(str(post_data["title"]))
+    content = str(post_data["content"])
+    tags = ", ".join(post_data["tags"])
+
+    home = HomePage(user_page, base_url)
+    home.open()
+    home.create_post()
+
+    form = PostFormPage(user_page, base_url)
+    form.fill_form(title, content, tags)
+    with user_page.expect_navigation(wait_until="networkidle"):
+        form.publish()
+    current_url = user_page.url
+    assert "new" not in current_url, f"Still on form page: {current_url}"
+    slug = current_url.rstrip("/").split("/")[-1]
+
+    user_page.wait_for_selector('[data-testid="post-detail-title"]')
+    detail = PostDetailPage(user_page, base_url, slug)
+    assert detail.get_title() == title
+
+    # Click "Write a Comment" button to show the form
+    user_page.locator('[data-testid="btn-show-comment-form"]').click()
+    user_page.wait_for_selector('[data-testid="form-create-comment"]', state="visible")
+
+    # Write a top-level comment
+    comment_text = "This is a top-level comment with enough length for testing."
+    user_page.locator('[data-testid="input-comment-content"]').fill(comment_text)
+
+    # Submit the comment
+    user_page.locator('[data-testid="submit-comment"]').click()
+
+    # Page should reload after successful comment creation
+    user_page.wait_for_selector('[data-testid="comments-section"]', timeout=15000)
+
+    # Verify the top-level comment appears
+    comment_locator = user_page.locator('[data-testid^="comment-content-"]', has_text=comment_text)
+    expect(comment_locator.first).to_be_visible(timeout=10000)
+
+    # Get the comment ID for the reply button
+    top_comment = user_page.locator('[data-testid^="comment-"][data-comment-id]').first
+    comment_id = top_comment.get_attribute("data-comment-id")
+
+    # Click Reply on the top-level comment
+    reply_btn = user_page.locator(f'[data-testid="btn-comment-reply-{comment_id}"]')
+    reply_btn.click()
+
+    # The comment form should appear with reply info
+    user_page.wait_for_selector('[data-testid="comment-form-help"]', state="visible")
+
+    # Write a reply
+    reply_text = "This is a reply to the comment."
+    user_page.locator('[data-testid="input-comment-content"]').fill(reply_text)
+
+    # Submit the reply
+    user_page.locator('[data-testid="submit-comment"]').click()
+
+    # Page should reload
+    user_page.wait_for_selector('[data-testid="comments-section"]', timeout=15000)
+
+    # Verify the reply appears in the comment-replies section under the parent
+    replies_section = user_page.locator(f'[data-testid="comment-replies-{comment_id}"]')
+    expect(replies_section).to_be_visible(timeout=10000)
+
+    # Verify reply text is visible within the replies section
+    reply_in_section = replies_section.locator(
+        '[data-testid^="comment-content-"]', has_text=reply_text
+    )
+    expect(reply_in_section).to_be_visible(timeout=5000)
+
+    # Get the reply's comment ID for the nested reply
+    reply_element = replies_section.locator('[data-testid^="comment-"][data-comment-id]').first
+    reply_id = reply_element.get_attribute("data-comment-id")
+
+    # Click Reply on the reply (nested reply)
+    user_page.locator(f'[data-testid="btn-comment-reply-{reply_id}"]').click()
+    user_page.wait_for_selector('[data-testid="comment-form-help"]', state="visible")
+
+    # Write a nested reply
+    nested_text = "This is a reply to the reply."
+    user_page.locator('[data-testid="input-comment-content"]').fill(nested_text)
+
+    # Submit the nested reply
+    user_page.locator('[data-testid="submit-comment"]').click()
+
+    # Page should reload
+    user_page.wait_for_selector('[data-testid="comments-section"]', timeout=15000)
+
+    # Verify the nested reply appears in the comment-replies section under the reply
+    nested_replies = user_page.locator(f'[data-testid="comment-replies-{reply_id}"]')
+    expect(nested_replies).to_be_visible(timeout=10000)
+
+    # Verify nested reply text is visible
+    nested_in_section = nested_replies.locator(
+        '[data-testid^="comment-content-"]', has_text=nested_text
+    )
+    expect(nested_in_section).to_be_visible(timeout=5000)
+
+
+@pytest.mark.e2e
+def test_guest_cannot_comment(
+    guest_page: Page,
+    user_page: Page,
+    base_url: str,
+) -> None:
+    """Test TC-E2E-112: Guest cannot see the comment form.
+
+    Steps:
+        1. User creates and publishes a post.
+        2. Guest opens the post detail page.
+        3. Verify guest cannot see the "Write a Comment" button.
+
+    Args:
+        guest_page: Unauthenticated Playwright page.
+        user_page: Playwright page authenticated as regular user.
+        base_url: Application base URL.
+    """
+    generator = PostDataGenerator()
+    post_data = generator.generate_post()
+    title = _unique_title(str(post_data["title"]))
+    content = str(post_data["content"])
+    tags = ", ".join(post_data["tags"])
+
+    home = HomePage(user_page, base_url)
+    home.open()
+    home.create_post()
+
+    form = PostFormPage(user_page, base_url)
+    form.fill_form(title, content, tags)
+    with user_page.expect_navigation(wait_until="networkidle"):
+        form.publish()
+    current_url = user_page.url
+    assert "new" not in current_url, f"Still on form page: {current_url}"
+    slug = current_url.rstrip("/").split("/")[-1]
+
+    # Guest opens the published post
+    guest_detail = PostDetailPage(guest_page, base_url, slug)
+    guest_detail.open()
+    guest_page.wait_for_selector('[data-testid="post-detail-title"]')
+
+    # Verify guest cannot see comment form or button
+    comment_btn = guest_page.locator('[data-testid="btn-show-comment-form"]')
+    expect(comment_btn).to_have_count(0)

tests/e2e/test_likes.py

@@ -0,0 +1,183 @@
+"""End-to-end tests for post likes via web UI.
+
+Tests the like/unlike toggle flow, multi-user like isolation,
+and guest authentication redirect.
+"""
+
+from __future__ import annotations
+
+import uuid
+
+import pytest
+from playwright.sync_api import Page, expect
+from pytfm.generators import PostDataGenerator
+
+from tests.e2e.pages import HomePage, PostDetailPage, PostFormPage
+
+
+def _unique_title(base: str) -> str:
+    """Append a short UUID to a title to avoid slug collisions."""
+    return f"{base} {uuid.uuid4().hex[:8]}"
+
+
+@pytest.mark.e2e
+def test_like_unlike_flow(
+    user_page: Page,
+    base_url: str,
+) -> None:
+    """Test like/unlike toggle through the web UI.
+
+    Steps:
+        1. Create and publish a post.
+        2. Verify initial like count is 0.
+        3. Click the like button.
+        4. Verify like count becomes 1.
+        5. Click the like button again.
+        6. Verify like count returns to 0.
+
+    Args:
+        user_page: Playwright page authenticated as regular user.
+        base_url: Application base URL.
+    """
+    generator = PostDataGenerator()
+    post_data = generator.generate_post()
+    title = _unique_title(str(post_data["title"]))
+    content = str(post_data["content"])
+    tags = ", ".join(post_data["tags"])
+
+    home = HomePage(user_page, base_url)
+    home.open()
+    home.create_post()
+
+    form = PostFormPage(user_page, base_url)
+    form.fill_form(title, content, tags)
+    with user_page.expect_navigation(wait_until="networkidle"):
+        form.publish()
+    current_url = user_page.url
+    assert "new" not in current_url, f"Still on form page: {current_url}"
+    slug = current_url.rstrip("/").split("/")[-1]
+
+    user_page.wait_for_selector('[data-testid="post-detail-title"]')
+    detail = PostDetailPage(user_page, base_url, slug)
+    assert detail.get_title() == title
+
+    # Initial like count should be 0 for a new post
+    assert detail.get_like_count() == 0
+
+    # Like the post
+    detail.click_like()
+    expect(user_page.locator("#like-count")).to_have_text("1", timeout=15000)
+
+    # Unlike the post
+    detail.click_like()
+    expect(user_page.locator("#like-count")).to_have_text("0", timeout=15000)
+
+
+@pytest.mark.e2e
+def test_multiple_users_can_like(
+    user_page: Page,
+    user2_page: Page,
+    base_url: str,
+) -> None:
+    """Test that two users can independently like the same post.
+
+    Steps:
+        1. User creates and publishes a post.
+        2. User likes the post (count becomes 1).
+        3. User2 opens the same post (sees count=1).
+        4. User2 clicks like (count becomes 2).
+
+    Args:
+        user_page: Playwright page authenticated as first regular user.
+        user2_page: Playwright page authenticated as second regular user.
+        base_url: Application base URL.
+    """
+    generator = PostDataGenerator()
+    post_data = generator.generate_post()
+    title = _unique_title(str(post_data["title"]))
+    content = str(post_data["content"])
+    tags = ", ".join(post_data["tags"])
+
+    home = HomePage(user_page, base_url)
+    home.open()
+    home.create_post()
+
+    form = PostFormPage(user_page, base_url)
+    form.fill_form(title, content, tags)
+    with user_page.expect_navigation(wait_until="networkidle"):
+        form.publish()
+    current_url = user_page.url
+    assert "new" not in current_url, f"Still on form page: {current_url}"
+    slug = current_url.rstrip("/").split("/")[-1]
+
+    user_page.wait_for_selector('[data-testid="post-detail-title"]')
+    detail = PostDetailPage(user_page, base_url, slug)
+    assert detail.get_title() == title
+
+    # User likes the post
+    assert detail.get_like_count() == 0
+    detail.click_like()
+    expect(user_page.locator("#like-count")).to_have_text("1", timeout=15000)
+
+    # Verify like_count persists after page reload
+    user_page.reload(wait_until="networkidle")
+    user_page.wait_for_selector('[data-testid="post-detail-title"]')
+    assert detail.get_like_count() == 1
+
+    # User2 opens same post and likes
+    user2_detail = PostDetailPage(user2_page, base_url, slug)
+    user2_detail.open()
+    user2_page.wait_for_selector('[data-testid="post-detail-title"]')
+    assert user2_detail.get_like_count() == 1
+
+    user2_detail.click_like()
+    expect(user2_page.locator("#like-count")).to_have_text("2", timeout=15000)
+
+
+@pytest.mark.e2e
+def test_guest_redirect_on_like(
+    user_page: Page,
+    guest_page: Page,
+    base_url: str,
+) -> None:
+    """Test that unauthenticated guests are redirected to login when liking.
+
+    Steps:
+        1. User creates and publishes a post.
+        2. Guest opens the post detail page.
+        3. Guest clicks the like button.
+        4. Guest is redirected to the development login page.
+
+    Args:
+        user_page: Playwright page authenticated as regular user.
+        guest_page: Unauthenticated Playwright page.
+        base_url: Application base URL.
+    """
+    generator = PostDataGenerator()
+    post_data = generator.generate_post()
+    title = _unique_title(str(post_data["title"]))
+    content = str(post_data["content"])
+    tags = ", ".join(post_data["tags"])
+
+    home = HomePage(user_page, base_url)
+    home.open()
+    home.create_post()
+
+    form = PostFormPage(user_page, base_url)
+    form.fill_form(title, content, tags)
+    with user_page.expect_navigation(wait_until="networkidle"):
+        form.publish()
+    current_url = user_page.url
+    assert "new" not in current_url, f"Still on form page: {current_url}"
+    slug = current_url.rstrip("/").split("/")[-1]
+
+    # Guest opens the published post
+    guest_detail = PostDetailPage(guest_page, base_url, slug)
+    guest_detail.open()
+    guest_page.wait_for_selector('[data-testid="post-detail-title"]')
+
+    # Guest clicks like -> should be redirected to dev login page
+    with guest_page.expect_navigation(wait_until="networkidle", timeout=15000):
+        guest_detail.click_like()
+
+    assert "dev-login" in guest_page.url

tests/unit/application/test_create_comment.py

@@ -0,0 +1,128 @@
+"""Tests for CreateCommentUseCase.
+
+This module tests comment creation use case covering top-level comments,
+replies to existing comments, and post-not-found scenarios.
+"""
+
+from unittest.mock import AsyncMock, Mock
+from uuid import uuid4
+
+import pytest
+
+from app.application.use_cases.create_comment import CreateCommentUseCase
+from app.domain.entities import Post
+from app.domain.exceptions import NotFoundException
+
+
+@pytest.fixture
+def test_post() -> Post:
+    """Create a test post for comment tests."""
+    return Post.create(
+        title_str="Commentable Post",
+        content_str="This post will receive comments. Enough length here.",
+        author_id="user-123",
+        tags=["test"],
+    )
+
+
+class TestCreateCommentUseCase:
+    """Tests for CreateCommentUseCase.
+
+    Covers TC-UNIT-832 through TC-UNIT-834.
+    """
+
+    @pytest.mark.asyncio
+    async def test_create_comment_on_post(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+        test_post: Post,
+    ) -> None:
+        """Test creating a top-level comment on a post.
+
+        TC-UNIT-832: Positive — create top-level comment.
+        """
+        mock_comment_repository = AsyncMock()
+        mock_post_repository.get_by_id = AsyncMock(return_value=test_post)
+
+        use_case = CreateCommentUseCase(
+            post_repo=mock_post_repository,
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        result = await use_case.execute(
+            post_id=test_post.id,
+            author_id="user-456",
+            content="Great post! Thanks for sharing.",
+        )
+
+        assert result.post_id == test_post.id
+        assert result.author_id == "user-456"
+        assert result.content == "Great post! Thanks for sharing."
+        assert result.parent_id is None
+        assert result.like_count == 0
+        mock_comment_repository.add.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_create_comment_reply(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+        test_post: Post,
+    ) -> None:
+        """Test creating a reply to an existing comment.
+
+        TC-UNIT-833: Positive — reply to comment with parent_id.
+        """
+        mock_comment_repository = AsyncMock()
+        mock_post_repository.get_by_id = AsyncMock(return_value=test_post)
+        parent_id = uuid4()
+
+        use_case = CreateCommentUseCase(
+            post_repo=mock_post_repository,
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        result = await use_case.execute(
+            post_id=test_post.id,
+            author_id="user-456",
+            content="This is a reply.",
+            parent_id=parent_id,
+        )
+
+        assert result.parent_id == parent_id
+        assert result.post_id == test_post.id
+        mock_comment_repository.add.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_create_comment_post_not_found(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+    ) -> None:
+        """Test creating a comment on a non-existent post.
+
+        TC-UNIT-834: Negative — post not found.
+        """
+        mock_comment_repository = AsyncMock()
+        mock_post_repository.get_by_id = AsyncMock(return_value=None)
+
+        use_case = CreateCommentUseCase(
+            post_repo=mock_post_repository,
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        with pytest.raises(NotFoundException):
+            await use_case.execute(
+                post_id=uuid4(),
+                author_id="user-456",
+                content="Comment on missing post.",
+            )
+
+        mock_comment_repository.add.assert_not_called()
+        mock_transaction_manager.commit.assert_not_called()

tests/unit/application/test_delete_comment.py

@@ -0,0 +1,81 @@
+"""Tests for DeleteCommentUseCase.
+
+This module tests the comment deletion use case covering own comment
+deletion, admin deletion, and not-found scenarios.
+"""
+
+from unittest.mock import AsyncMock
+from uuid import uuid4
+
+import pytest
+
+from app.application.use_cases.delete_comment import DeleteCommentUseCase
+from app.domain.entities.comment import Comment
+from app.domain.exceptions import NotFoundException
+
+
+class TestDeleteCommentUseCase:
+    """Tests for DeleteCommentUseCase.
+
+    Covers TC-UNIT-836 and TC-UNIT-837.
+    """
+
+    @pytest.mark.asyncio
+    async def test_delete_own_comment(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test deleting own comment.
+
+        TC-UNIT-836: Positive — user deletes own comment.
+        """
+        post_id = uuid4()
+        author_id = "user-123"
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str="Comment to delete.",
+        )
+
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=comment)
+        mock_comment_repository.delete = AsyncMock()
+
+        use_case = DeleteCommentUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        await use_case.execute(
+            comment_id=comment.id,
+            user_id=author_id,
+        )
+
+        mock_comment_repository.delete.assert_called_once_with(comment.id)
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_delete_comment_not_found(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test deleting a non-existent comment.
+
+        TC-UNIT-837: Negative — comment not found.
+        """
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=None)
+
+        use_case = DeleteCommentUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        with pytest.raises(NotFoundException):
+            await use_case.execute(
+                comment_id=uuid4(),
+                user_id="user-123",
+            )
+
+        mock_comment_repository.delete.assert_not_called()
+        mock_transaction_manager.commit.assert_not_called()

tests/unit/application/test_list_comments.py

@@ -0,0 +1,59 @@
+"""Tests for ListCommentsUseCase.
+
+This module tests the comment listing use case covering retrieval
+of comments by post ID.
+"""
+
+from unittest.mock import AsyncMock, Mock
+from uuid import uuid4
+
+import pytest
+
+from app.application.use_cases.list_comments import ListCommentsUseCase
+from app.domain.entities.comment import Comment
+
+
+class TestListCommentsUseCase:
+    """Tests for ListCommentsUseCase.
+
+    Covers TC-UNIT-835.
+    """
+
+    @pytest.mark.asyncio
+    async def test_list_comments_by_post(
+        self,
+        mock_transaction_manager: Mock,
+    ) -> None:
+        """Test listing comments for a post.
+
+        TC-UNIT-835: Positive — returns comments for given post_id.
+        """
+        post_id = uuid4()
+        author_id = "user-123"
+
+        comments = [
+            Comment.create(
+                post_id=post_id,
+                author_id=author_id,
+                content_str="First comment.",
+            ),
+            Comment.create(
+                post_id=post_id,
+                author_id="user-456",
+                content_str="Second comment.",
+            ),
+        ]
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_post = AsyncMock(return_value=comments)
+
+        use_case = ListCommentsUseCase(
+            comment_repo=mock_comment_repository,
+        )
+
+        result = await use_case.execute(post_id=post_id)
+
+        assert len(result) == 2
+        assert result[0].post_id == post_id
+        assert result[0].author_id == author_id
+        assert result[1].author_id == "user-456"
+        mock_comment_repository.get_by_post.assert_called_once_with(post_id)

tests/unit/application/test_toggle_comment_like.py

@@ -0,0 +1,119 @@
+"""Tests for ToggleCommentLikeUseCase.
+
+This module tests the comment like/unlike toggle use case covering
+first-time like, unlike, and comment-not-found scenarios.
+"""
+
+from unittest.mock import AsyncMock
+from uuid import uuid4
+
+import pytest
+
+from app.application.use_cases.toggle_comment_like import ToggleCommentLikeUseCase
+from app.domain.entities.comment import Comment
+from app.domain.entities.comment_like import CommentLike
+from app.domain.exceptions import NotFoundException
+
+
+class TestToggleCommentLikeUseCase:
+    """Tests for ToggleCommentLikeUseCase.
+
+    Covers TC-UNIT-838 through TC-UNIT-840.
+    """
+
+    @pytest.mark.asyncio
+    async def test_like_comment_first_time(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test liking a comment for the first time.
+
+        TC-UNIT-838: Positive — like first time.
+        """
+        post_id = uuid4()
+        comment = Comment.create(
+            post_id=post_id,
+            author_id="user-123",
+            content_str="Nice post!",
+        )
+
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=comment)
+        mock_comment_repository.get_like = AsyncMock(return_value=None)
+        mock_comment_repository.add_like = AsyncMock()
+        mock_comment_repository.remove_like = AsyncMock()
+        mock_comment_repository.update = AsyncMock()
+
+        use_case = ToggleCommentLikeUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        result = await use_case.execute(comment.id, "user-456")
+
+        assert result.like_count == 1
+        mock_comment_repository.add_like.assert_called_once()
+        mock_comment_repository.remove_like.assert_not_called()
+        mock_comment_repository.update.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_unlike_comment_already_liked(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test unliking a comment that is already liked.
+
+        TC-UNIT-839: Positive — unlike (already liked).
+        """
+        post_id = uuid4()
+        comment = Comment.create(
+            post_id=post_id,
+            author_id="user-123",
+            content_str="Nice post!",
+        )
+        existing_like = CommentLike(comment_id=comment.id, liked_by="user-456")
+
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=comment)
+        mock_comment_repository.get_like = AsyncMock(return_value=existing_like)
+        mock_comment_repository.add_like = AsyncMock()
+        mock_comment_repository.remove_like = AsyncMock()
+        mock_comment_repository.update = AsyncMock()
+
+        use_case = ToggleCommentLikeUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        result = await use_case.execute(comment.id, "user-456")
+
+        assert result.like_count == 0
+        mock_comment_repository.remove_like.assert_called_once()
+        mock_comment_repository.add_like.assert_not_called()
+        mock_comment_repository.update.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_like_comment_not_found(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test liking a non-existent comment.
+
+        TC-UNIT-840: Negative — comment not found.
+        """
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=None)
+
+        use_case = ToggleCommentLikeUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        with pytest.raises(NotFoundException):
+            await use_case.execute(uuid4(), "user-456")
+
+        mock_comment_repository.add_like.assert_not_called()
+        mock_comment_repository.remove_like.assert_not_called()
+        mock_transaction_manager.commit.assert_not_called()

tests/unit/application/test_toggle_like.py

@@ -0,0 +1,170 @@
+"""Tests for TogglePostLikeUseCase.
+
+This module tests the like/unlike toggle use case covering
+first-time like, unlike, post-not-found, guest access, and
+identity isolation scenarios.
+"""
+
+from unittest.mock import AsyncMock, Mock
+from uuid import uuid4
+
+import pytest
+
+from app.application.use_cases.toggle_like import TogglePostLikeUseCase
+from app.domain.entities import Post
+from app.domain.entities.like import PostLike
+from app.domain.exceptions import NotFoundException
+
+
+@pytest.fixture
+def test_post() -> Post:
+    """Create a test post for like tests."""
+    return Post.create(
+        title_str="Likeable Post",
+        content_str="This post will be liked and unliked. Enough length here.",
+        author_id="user-123",
+        tags=["test"],
+    )
+
+
+class TestTogglePostLikeUseCase:
+    """Tests for TogglePostLikeUseCase.
+
+    Covers TC-UNIT-822 through TC-UNIT-825 and TC-UNIT-828.
+    """
+
+    @pytest.mark.asyncio
+    async def test_like_post_first_time(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+        test_post: Post,
+    ) -> None:
+        """Test toggling like on a post for the first time.
+
+        TC-UNIT-822: Positive — like first time.
+        """
+        mock_post_repository.get_by_id = AsyncMock(return_value=test_post)
+        mock_post_repository.get_like = AsyncMock(return_value=None)
+        mock_post_repository.add_like = AsyncMock()
+        mock_post_repository.remove_like = AsyncMock()
+        mock_post_repository.update = AsyncMock()
+
+        use_case = TogglePostLikeUseCase(mock_post_repository, mock_transaction_manager)
+
+        result = await use_case.execute(test_post.id, "user-123")
+
+        assert result.like_count == 1
+        mock_post_repository.add_like.assert_called_once()
+        mock_post_repository.remove_like.assert_not_called()
+        mock_post_repository.update.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_unlike_post_already_liked(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+        test_post: Post,
+    ) -> None:
+        """Test toggling like on a post that is already liked.
+
+        TC-UNIT-823: Positive — unlike (already liked).
+        """
+        existing_like = PostLike(post_id=test_post.id, liked_by="user-123")
+
+        mock_post_repository.get_by_id = AsyncMock(return_value=test_post)
+        mock_post_repository.get_like = AsyncMock(return_value=existing_like)
+        mock_post_repository.add_like = AsyncMock()
+        mock_post_repository.remove_like = AsyncMock()
+        mock_post_repository.update = AsyncMock()
+
+        use_case = TogglePostLikeUseCase(mock_post_repository, mock_transaction_manager)
+
+        result = await use_case.execute(test_post.id, "user-123")
+
+        assert result.like_count == 0
+        mock_post_repository.remove_like.assert_called_once()
+        mock_post_repository.add_like.assert_not_called()
+        mock_post_repository.update.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_like_post_not_found(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+    ) -> None:
+        """Test toggling like on a non-existent post.
+
+        TC-UNIT-824: Negative — post not found.
+        """
+        mock_post_repository.get_by_id = AsyncMock(return_value=None)
+
+        use_case = TogglePostLikeUseCase(mock_post_repository, mock_transaction_manager)
+
+        with pytest.raises(NotFoundException):
+            await use_case.execute(uuid4(), "user-123")
+
+        mock_post_repository.add_like.assert_not_called()
+        mock_post_repository.remove_like.assert_not_called()
+        mock_transaction_manager.commit.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_like_as_guest_with_device_id(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+        test_post: Post,
+    ) -> None:
+        """Test toggling like as a guest using device_id.
+
+        TC-UNIT-825: Positive — guest via device_id.
+        """
+        device_id = "device-abc-123"
+        mock_post_repository.get_by_id = AsyncMock(return_value=test_post)
+        mock_post_repository.get_like = AsyncMock(return_value=None)
+        mock_post_repository.add_like = AsyncMock()
+        mock_post_repository.remove_like = AsyncMock()
+        mock_post_repository.update = AsyncMock()
+
+        use_case = TogglePostLikeUseCase(mock_post_repository, mock_transaction_manager)
+
+        result = await use_case.execute(test_post.id, device_id)
+
+        assert result.like_count == 1
+        added_like = mock_post_repository.add_like.call_args[0][0]
+        assert added_like.liked_by == device_id
+        assert added_like.post_id == test_post.id
+
+    @pytest.mark.asyncio
+    async def test_two_users_can_both_like(
+        self,
+        mock_post_repository: Mock,
+        mock_transaction_manager: Mock,
+        test_post: Post,
+    ) -> None:
+        """Test that two different users can both like the same post.
+
+        TC-UNIT-828: Positive — identity isolation.
+        Both likes are counted independently.
+        """
+        mock_post_repository.get_by_id = AsyncMock(return_value=test_post)
+        mock_post_repository.get_like = AsyncMock(return_value=None)
+        mock_post_repository.add_like = AsyncMock()
+        mock_post_repository.remove_like = AsyncMock()
+        mock_post_repository.update = AsyncMock()
+
+        use_case = TogglePostLikeUseCase(mock_post_repository, mock_transaction_manager)
+
+        result1 = await use_case.execute(test_post.id, "user-123")
+        assert result1.like_count == 1
+
+        mock_post_repository.add_like.reset_mock()
+        mock_post_repository.update.reset_mock()
+        mock_transaction_manager.commit.reset_mock()
+
+        result2 = await use_case.execute(test_post.id, "user-456")
+        assert result2.like_count == 2
+
+        assert mock_post_repository.add_like.call_count == 1

tests/unit/domain/test_comment_entity.py

@@ -0,0 +1,98 @@
+"""Tests for Comment domain entity.
+
+This module tests the Comment entity creation, parent_id support,
+and BaseEntity integration.
+"""
+
+from uuid import UUID, uuid4
+
+from app.domain.entities.comment import Comment
+from app.domain.value_objects.comment_content import CommentContent
+
+
+class TestCommentEntity:
+    """Tests for the Comment domain entity.
+
+    Covers TC-UNIT-829 and TC-UNIT-830.
+    """
+
+    def test_comment_creation(self) -> None:
+        """Test creating a top-level Comment with valid attributes.
+
+        TC-UNIT-829: Positive — create Comment instance.
+
+        Expected:
+            - post_id matches input
+            - author_id matches input
+            - content is CommentContent with correct value
+            - id is a valid UUID
+            - parent_id is None
+            - like_count is 0
+            - created_at is set
+        """
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        author_id = "user-123"
+        content_text = "This is a comment with **Markdown** support."
+
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str=content_text,
+        )
+
+        assert comment.post_id == post_id
+        assert comment.author_id == author_id
+        assert isinstance(comment.content, CommentContent)
+        assert comment.content.value == content_text
+        assert isinstance(comment.id, UUID)
+        assert comment.parent_id is None
+        assert comment.like_count == 0
+        assert comment.created_at is not None
+
+    def test_comment_with_parent(self) -> None:
+        """Test creating a reply Comment with parent_id.
+
+        TC-UNIT-830: Positive — create Comment with parent_id.
+
+        Expected:
+            - parent_id matches the provided parent comment ID.
+            - All other attributes set correctly.
+        """
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        parent_id = uuid4()
+        author_id = "user-456"
+        content_text = "This is a reply to another comment."
+
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str=content_text,
+            parent_id=parent_id,
+        )
+
+        assert comment.parent_id == parent_id
+        assert comment.post_id == post_id
+        assert comment.author_id == author_id
+        assert comment.content.value == content_text
+        assert comment.like_count == 0
+
+    def test_comment_to_dict(self) -> None:
+        """Test Comment to_dict serialization."""
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        author_id = "user-123"
+        content_text = "Comment with serialization test."
+
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str=content_text,
+        )
+        data = comment.to_dict()
+
+        assert data["post_id"] == str(post_id)
+        assert data["author_id"] == author_id
+        assert data["content"] == content_text
+        assert "id" in data
+        assert "created_at" in data
+        assert data["parent_id"] is None
+        assert data["like_count"] == 0

tests/unit/domain/test_comment_like_entity.py

@@ -0,0 +1,50 @@
+"""Tests for CommentLike domain entity.
+
+This module tests the CommentLike entity creation, attributes,
+and BaseEntity integration.
+"""
+
+from uuid import UUID
+
+from app.domain.entities.comment_like import CommentLike
+
+
+class TestCommentLikeEntity:
+    """Tests for the CommentLike domain entity.
+
+    Covers TC-UNIT-831.
+    """
+
+    def test_comment_like_creation(self) -> None:
+        """Test creating a CommentLike with valid attributes.
+
+        TC-UNIT-831: Positive — create CommentLike instance.
+
+        Expected:
+            - comment_id matches input
+            - liked_by matches input
+            - id is a valid UUID
+            - created_at is set
+        """
+        comment_id = UUID("00000000-0000-0000-0000-000000000001")
+        liked_by = "user-123"
+
+        like = CommentLike(comment_id=comment_id, liked_by=liked_by)
+
+        assert like.comment_id == comment_id
+        assert like.liked_by == liked_by
+        assert isinstance(like.id, UUID)
+        assert like.created_at is not None
+
+    def test_comment_like_to_dict(self) -> None:
+        """Test CommentLike to_dict serialization."""
+        comment_id = UUID("00000000-0000-0000-0000-000000000001")
+        liked_by = "device-abc-123"
+
+        like = CommentLike(comment_id=comment_id, liked_by=liked_by)
+        data = like.to_dict()
+
+        assert data["comment_id"] == str(comment_id)
+        assert data["liked_by"] == liked_by
+        assert "id" in data
+        assert "created_at" in data

tests/unit/domain/test_entities.py

@@ -128,6 +128,19 @@ class TestPost:
         assert "created_at" in data
         assert "updated_at" in data
 
+    def test_like_count_defaults_to_zero(self) -> None:
+        """Test that a new post has like_count defaulting to 0.
+
+        TC-UNIT-827: Positive — like_count defaults to zero on creation.
+        """
+        post = Post.create(
+            title_str="Test Post",
+            content_str="This is test content that is long enough",
+            author_id="user-123",
+        )
+
+        assert post.like_count == 0
+
     def test_base_entity_eq_and_hash(self) -> None:
         """Test BaseEntity equality and hash directly."""
         from app.domain.entities.base import BaseEntity

tests/unit/domain/test_like_entity.py

@@ -0,0 +1,50 @@
+"""Tests for PostLike domain entity.
+
+This module tests the PostLike entity creation, attributes,
+and BaseEntity integration.
+"""
+
+from uuid import UUID
+
+from app.domain.entities.like import PostLike
+
+
+class TestPostLikeEntity:
+    """Tests for the PostLike domain entity.
+
+    Covers TC-UNIT-826: PostLike entity valid creation.
+    """
+
+    def test_post_like_creation(self) -> None:
+        """Test creating a PostLike with valid attributes.
+
+        TC-UNIT-826: Positive — create PostLike instance.
+
+        Expected:
+            - post_id matches input
+            - liked_by matches input
+            - id is a valid UUID
+            - created_at is set
+        """
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        liked_by = "user-123"
+
+        like = PostLike(post_id=post_id, liked_by=liked_by)
+
+        assert like.post_id == post_id
+        assert like.liked_by == liked_by
+        assert isinstance(like.id, UUID)
+        assert like.created_at is not None
+
+    def test_post_like_to_dict(self) -> None:
+        """Test PostLike to_dict serialization."""
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        liked_by = "device-abc-123"
+
+        like = PostLike(post_id=post_id, liked_by=liked_by)
+        data = like.to_dict()
+
+        assert data["post_id"] == str(post_id)
+        assert data["liked_by"] == liked_by
+        assert "id" in data
+        assert "created_at" in data

tests/unit/infrastructure/test_i18n.py

@@ -0,0 +1,196 @@
+"""Tests for i18n infrastructure.
+
+Covers TranslationService, locale detection helpers, and the
+convenience _() function.
+"""
+
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+import pytest
+
+from app.infrastructure.i18n.translator import (
+    DEFAULT_LOCALE,
+    TranslationService,
+    _,
+)
+from app.presentation.web.locale import (
+    _get_best_locale,
+    _parse_accept_language,
+    setup_locale_manager,
+)
+
+
+def _make_mock_request(
+    cookies: dict[str, str] | None = None,
+    headers: dict[str, str] | None = None,
+) -> MagicMock:
+    """Create a mock FastAPI Request with controlled state, cookies and headers."""
+    request = MagicMock()
+    request.state = SimpleNamespace()
+    request.cookies = cookies or {}
+    request.headers = headers or {}
+    return request
+
+
+class TestTranslationService:
+    """Test TranslationService get_text resolution and fallback chain."""
+
+    def test_get_text_returns_translation_for_existing_key(self) -> None:
+        """Test get_text returns the correct translation for a known key."""
+        ts = TranslationService()
+        result = ts.get_text("nav.home", "ru")
+        assert result == "Главная"
+
+    def test_get_text_returns_english_fallback_for_missing_key(self) -> None:
+        """Test get_text falls back to English when locale lacks the key."""
+        ts = TranslationService()
+        result = ts.get_text("nav.home", "fr")
+        assert result == "Accueil"
+
+    def test_get_text_returns_key_when_neither_locale_nor_en_has_it(self) -> None:
+        """Test get_text returns the key itself when no translation exists anywhere."""
+        ts = TranslationService()
+        result = ts.get_text("nonexistent.key", "de")
+        assert result == "nonexistent.key"
+
+    def test_get_text_returns_english_fallback_for_unknown_locale(self) -> None:
+        """Test get_text returns English when the requested locale does not exist."""
+        ts = TranslationService()
+        result = ts.get_text("nav.home", "zz")
+        assert result == "Home"
+
+    def test_get_text_returns_en_when_requested_locale_is_en(self) -> None:
+        """Test get_text returns English string when locale is en."""
+        ts = TranslationService()
+        result = ts.get_text("nav.home", "en")
+        assert result == "Home"
+
+    def test_get_text_returns_en_fallback_when_requested_locale_missing_key(self) -> None:
+        """Test get_text falls back to English when locale is unknown and key exists in en."""
+        ts = TranslationService()
+        result = ts.get_text("about.signed_in", "zz")
+        assert result == "Signed in as {username}."
+
+    def test_singleton_returns_same_instance(self) -> None:
+        """Test TranslationService.get_instance always returns the same instance."""
+        instance_a = TranslationService.get_instance()
+        instance_b = TranslationService.get_instance()
+        assert instance_a is instance_b
+
+    def test_singleton_shares_translations(self) -> None:
+        """Test that multiple calls via singleton share the same data."""
+        instance_a = TranslationService.get_instance()
+        instance_b = TranslationService.get_instance()
+        assert instance_a.translations is instance_b.translations
+
+
+class TestConvenienceFunction:
+    """Test the module-level _() convenience function."""
+
+    def test_convenience_function_returns_translation(self) -> None:
+        """Test _() returns correct translation for a given key."""
+        result = _("header.logo", "de")
+        assert result == "Blog"
+
+    def test_convenience_function_defaults_to_english(self) -> None:
+        """Test _() uses DEFAULT_LOCALE when no locale is given."""
+        result = _("nav.posts")
+        assert result == "Posts"
+        assert DEFAULT_LOCALE == "en"
+
+    def test_convenience_function_returns_key_on_missing(self) -> None:
+        """Test _() returns key when translation does not exist."""
+        result = _("utterly.fake.key", "fr")
+        assert result == "utterly.fake.key"
+
+
+class TestParseAcceptLanguage:
+    """Test the Accept-Language header parser."""
+
+    def test_empty_header_returns_empty_list(self) -> None:
+        """Test empty Accept-Language returns empty list."""
+        assert _parse_accept_language("") == []
+
+    def test_single_locale(self) -> None:
+        """Test a single locale code is returned as a single-element list."""
+        assert _parse_accept_language("fr") == ["fr"]
+
+    def test_multiple_locales_with_quality(self) -> None:
+        """Test multiple locales with q-values are returned in order."""
+        result = _parse_accept_language("fr, en;q=0.9, de;q=0.8")
+        assert result == ["fr", "en", "de"]
+
+    def test_region_code_strips_to_base_language(self) -> None:
+        """Test fr-FR is normalised to fr."""
+        assert _parse_accept_language("fr-FR") == ["fr"]
+
+    def test_complex_header(self) -> None:
+        """Test a realistic Accept-Language header with multiple locales."""
+        result = _parse_accept_language("ru-RU, ru;q=0.9, en;q=0.5")
+        assert result == ["ru", "ru", "en"]
+
+    def test_whitespace_around_locales(self) -> None:
+        """Test whitespace around locale codes is handled gracefully."""
+        result = _parse_accept_language("  en ,  fr;q=0.5  ")
+        assert result == ["en", "fr"]
+
+
+class TestGetBestLocale:
+    """Test locale detection from cookie and Accept-Language header."""
+
+    def test_cookie_takes_priority(self) -> None:
+        """Test cookie value is used when it is a supported locale."""
+        request = _make_mock_request(cookies={"locale": "de"}, headers={"accept-language": "ru"})
+        assert _get_best_locale(request) == "de"
+
+    def test_invalid_cookie_falls_back_to_header(self) -> None:
+        """Test unsupported locale in cookie falls back to Accept-Language."""
+        request = _make_mock_request(cookies={"locale": "zz"}, headers={"accept-language": "fr"})
+        assert _get_best_locale(request) == "fr"
+
+    def test_no_cookie_uses_accept_language(self) -> None:
+        """Test Accept-Language is used when no cookie is present."""
+        request = _make_mock_request(headers={"accept-language": "de"})
+        assert _get_best_locale(request) == "de"
+
+    def test_no_cookie_no_header_returns_default(self) -> None:
+        """Test default locale returned when neither cookie nor header matches."""
+        request = _make_mock_request()
+        assert _get_best_locale(request) == DEFAULT_LOCALE
+
+    def test_accept_language_unsupported_returns_default(self) -> None:
+        """Test unsupported language in header falls back to default."""
+        request = _make_mock_request(headers={"accept-language": "zh"})
+        assert _get_best_locale(request) == DEFAULT_LOCALE
+
+    def test_missing_cookie_key_uses_header(self) -> None:
+        """Test absent locale cookie is treated as no preference."""
+        request = _make_mock_request(cookies={"other": "val"}, headers={"accept-language": "ru"})
+        assert _get_best_locale(request) == "ru"
+
+
+class TestSetupLocaleManager:
+    """Test the middleware helper that sets locale on request state."""
+
+    @pytest.mark.asyncio
+    async def test_sets_locale_on_request_state(self) -> None:
+        """Test setup_locale_manager sets locale when not already present."""
+        request = _make_mock_request(headers={"accept-language": "fr"})
+        await setup_locale_manager(request)
+        assert request.state.locale == "fr"
+
+    @pytest.mark.asyncio
+    async def test_does_not_override_existing_locale(self) -> None:
+        """Test setup_locale_manager does not override an already-set locale."""
+        request = _make_mock_request(headers={"accept-language": "fr"})
+        request.state.locale = "de"
+        await setup_locale_manager(request)
+        assert request.state.locale == "de"
+
+    @pytest.mark.asyncio
+    async def test_default_locale_when_no_match(self) -> None:
+        """Test setup_locale_manager uses default when nothing matches."""
+        request = _make_mock_request()
+        await setup_locale_manager(request)
+        assert request.state.locale == DEFAULT_LOCALE