feat: add comments feature with nested replies and recursive rendering

Implement full comments system: domain entities (Comment, CommentLike),
value objects (CommentContent), use cases (CRUD, like toggle), SQLAlchemy
repository, API v1 endpoints, web UI with comment form and nested replies,
i18n translations (EN/RU/FR/DE), and E2E tests.

Fix nested reply (reply-to-reply) not displaying — the flat reply_comments
dict was only queried for top-level comment IDs, so deeply nested replies
were saved to DB (incrementing comment count) but never rendered. Switch
to a recursive Jinja2 macro that renders any nesting depth.

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

@@ -101,3 +101,4 @@ class PostResponseDTO:
     created_at: datetime
     updated_at: datetime
     like_count: int = 0
+    comment_count: int = 0