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"]