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/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.
+        """
+        ...