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/value_objects/__init__.py

@@ -5,8 +5,9 @@ immutable validated domain concepts.
 """
 
 from app.domain.value_objects.base import ValueObject
+from app.domain.value_objects.comment_content import CommentContent
 from app.domain.value_objects.content import Content
 from app.domain.value_objects.slug import Slug
 from app.domain.value_objects.title import Title
 
-__all__ = ["ValueObject", "Title", "Content", "Slug"]
+__all__ = ["ValueObject", "Title", "Content", "Slug", "CommentContent"]