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/__init__.py

@@ -4,11 +4,15 @@ This module re-exports all application use cases that implement
 business logic operations for the blog API.
 """
 
+from app.application.use_cases.create_comment import CreateCommentUseCase
 from app.application.use_cases.create_post import CreatePostUseCase
+from app.application.use_cases.delete_comment import DeleteCommentUseCase
 from app.application.use_cases.delete_post import DeletePostUseCase
 from app.application.use_cases.get_post import GetPostUseCase
+from app.application.use_cases.list_comments import ListCommentsUseCase
 from app.application.use_cases.list_posts import ListPostsUseCase
 from app.application.use_cases.publish_post import PublishPostUseCase
+from app.application.use_cases.toggle_comment_like import ToggleCommentLikeUseCase
 from app.application.use_cases.toggle_like import TogglePostLikeUseCase
 from app.application.use_cases.update_post import UpdatePostUseCase
 
@@ -20,4 +24,8 @@ __all__ = [
     "ListPostsUseCase",
     "PublishPostUseCase",
     "TogglePostLikeUseCase",
+    "CreateCommentUseCase",
+    "DeleteCommentUseCase",
+    "ListCommentsUseCase",
+    "ToggleCommentLikeUseCase",
 ]