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",
+]