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/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()