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/domain/entities/__init__.py

@@ -5,7 +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", "PostLike"]
+__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(),
+        }