gelbooru+moebooru: drop background prefetch from search, fetch on demand

Removes the asyncio.create_task(prefetch_batch) calls from
search() and get_post() in both clients. Tags are now fetched
ONLY when the user actually clicks a post (via ensure_categories
in the info panel path) or saves with a category-token template.

The background prefetch was the source of most of the complexity:
probe timing, early-exit bugs from partial composes racing with
on-click ensures, Rule34's slow probe blocking the prefetch
window. All gone.

New flow:
  search() → fast, returns posts with flat tags only
  click    → ensure_categories fires, ~200ms HTML scrape or
             batch API, categories arrive, signal re-renders
  re-click → instant (cache compose, no HTTP)
  save     → ensure in save_post_file, same path

The ~200ms per first-click is invisible during the image load.
The cache compounds across posts and sessions. The prefetch_batch
method stays in CategoryFetcher for potential future use but
nothing calls it from the hot path anymore.

booru_viewer/core/api/gelbooru.py

@@ -81,9 +81,6 @@ class GelbooruClient(BooruClient):
                     created_at=_parse_date(item.get("created_at")),
                 )
             )
-        if self.category_fetcher is not None:
-            import asyncio
-            asyncio.create_task(self.category_fetcher.prefetch_batch(posts))
         return posts
 
     @staticmethod

booru_viewer/core/api/moebooru.py

@@ -56,9 +56,6 @@ class MoebooruClient(BooruClient):
                     created_at=_parse_date(item.get("created_at")),
                 )
             )
-        if self.category_fetcher is not None:
-            import asyncio
-            asyncio.create_task(self.category_fetcher.prefetch_batch(posts))
         return posts
 
     async def get_post(self, post_id: int) -> Post | None: