docs: note generalizing the MB cache table to a shared ApiCache

The Last.fm browse page has the same live-external-call-per-load problem;
name the cache table/helper generically (ApiCache) so Last.fm rides the
same read-through, with a manual Refresh button and live own-state
annotation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/superpowers/plans/2026-07-14-musicbrainz-cache.md

@@ -151,6 +151,12 @@ unthrottled).
   prefer real on-disk tracks; this MB cache covers the non-owned / fallback tracklist path.
 
 ## Notes / deferred
+- **Name the table/helper generically** — the Last.fm browse page (`/lastfm`) has the same
+  problem (live external calls per load) and should ride the same read-through. Consider
+  `ApiCache(key, json, fetchedAt)` + `cached(...)` rather than MB-specific names, so Last.fm
+  keys (`lfm-top-artists:{user}:{period}:{page}`, etc.) share the table. See the Last.fm
+  caching todo. Only difference: Last.fm gets a manual **Refresh** button that busts its keys,
+  and own-state annotation stays live (cache only the raw Last.fm payload).
 - Redis deferred (see header). Reconsider only on measured PG-cache contention or a need for
   pub/sub / multi-instance.
 - `null`/negative caching gated on the Last.fm title-normalization todo landing first.