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/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,
+        )