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/__init__.py

@@ -4,14 +4,24 @@ This module re-exports all application layer components including
 DTOs, interfaces, and use cases for convenient importing.
 """
 
-from app.application.dtos import CreatePostDTO, PostResponseDTO, UpdatePostDTO
+from app.application.dtos import (
+    CommentResponseDTO,
+    CreateCommentDTO,
+    CreatePostDTO,
+    PostResponseDTO,
+    UpdatePostDTO,
+)
 from app.application.interfaces import TransactionManager
 from app.application.use_cases import (
+    CreateCommentUseCase,
     CreatePostUseCase,
+    DeleteCommentUseCase,
     DeletePostUseCase,
     GetPostUseCase,
+    ListCommentsUseCase,
     ListPostsUseCase,
     PublishPostUseCase,
+    ToggleCommentLikeUseCase,
     TogglePostLikeUseCase,
     UpdatePostUseCase,
 )
@@ -20,6 +30,8 @@ __all__ = [
     "CreatePostDTO",
     "UpdatePostDTO",
     "PostResponseDTO",
+    "CreateCommentDTO",
+    "CommentResponseDTO",
     "TransactionManager",
     "CreatePostUseCase",
     "GetPostUseCase",
@@ -28,4 +40,8 @@ __all__ = [
     "ListPostsUseCase",
     "PublishPostUseCase",
     "TogglePostLikeUseCase",
+    "CreateCommentUseCase",
+    "DeleteCommentUseCase",
+    "ListCommentsUseCase",
+    "ToggleCommentLikeUseCase",
 ]