Three changes:
1. _make_client passes db=self._db, site_id=s.id so Gelbooru and
Moebooru clients get a CategoryFetcher attached via the factory.
2. _on_post_activated calls _ensure_post_categories_async(post)
after setting up the preview. If the post has empty categories
(background prefetch hasn't reached it yet, or cache miss),
this schedules ensure_categories on the async loop. When it
completes, it emits categories_updated via the Qt signal.
3. _on_categories_updated slot re-renders the info panel and
preview pane tag display when the currently-selected post's
categories arrive. Stale updates (user clicked a different post
before the fill completed) are silently dropped by the post.id
check.