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/presentation/templates/pages/index.html

@@ -55,6 +55,9 @@
             <span class="post-card-meta-item" data-testid="like-count-{{ post.id }}">
                 👍 {{ post.like_count }}
             </span>
+            <span class="post-card-meta-item" data-testid="comment-count-{{ post.id }}">
+                💬 {{ post.comment_count }}
+            </span>
         </div>
         
         <div class="post-card-content" data-testid="post-content-preview-{{ post.id }}">