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/__init__.py

@@ -4,7 +4,7 @@ This module re-exports all domain layer components including
 entities, value objects, repositories, and exceptions.
 """
 
-from app.domain.entities import BaseEntity, Post, PostLike
+from app.domain.entities import BaseEntity, Comment, CommentLike, Post, PostLike
 from app.domain.exceptions import (
     AlreadyExistsException,
     DomainException,
@@ -13,19 +13,22 @@ from app.domain.exceptions import (
     UnauthorizedException,
     ValidationException,
 )
-from app.domain.repositories import PostRepository, Repository
+from app.domain.repositories import CommentRepository, PostRepository, Repository
 from app.domain.value_objects import Content, Slug, Title, ValueObject
 
 __all__ = [
     "BaseEntity",
     "Post",
     "PostLike",
+    "Comment",
+    "CommentLike",
     "ValueObject",
     "Title",
     "Content",
     "Slug",
     "Repository",
     "PostRepository",
+    "CommentRepository",
     "DomainException",
     "ValidationException",
     "NotFoundException",