feat(worker): share the MusicBrainz cache from scans/discovery

The worker's MB browser (discography + artist search, used by monitor sweeps,
library scans, and Last.fm discovery) re-fetched every time. Route it through
the same ApiCache table + logical keys the web app uses, so a discography one
process fetched warms the other, and repeat lookups stop hitting MB.

- apicache.py cached_api(): read-through, serve-stale-on-outage, never caches
  null, best-effort (a cache-DB error degrades to a live fetch, never breaks
  scan/sweep/discovery). Its own DEDICATED autocommit connection so it can't
  commit the worker's in-flight transaction, and autocommit avoids freezing
  Postgres now() (which would break TTL math).
- CachingMbBrowser decorates MusicBrainzBrowser, serializing to the SAME
  camelCase JSON shape the web writes (round-trip tested). registry.build_browser
  wraps it when a DSN is present; the Last.fm source's browser rides it too.
- Hourly prune drops ApiCache rows > 90d so the table stays bounded.
- worker 213 tests / 7-skip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/conftest.py

@@ -30,6 +30,7 @@ def conn():
         cur.execute('DELETE FROM "DiscoverySuggestion"')
         cur.execute('DELETE FROM "DiscoverySeedContribution"')
         cur.execute('DELETE FROM "ScanWorkItem"')
+        cur.execute('DELETE FROM "ApiCache"')
         cur.execute('DELETE FROM "Config"')
     connection.commit()
     yield connection
@@ -47,6 +48,7 @@ def conn():
         cur.execute('DELETE FROM "DiscoverySuggestion"')
         cur.execute('DELETE FROM "DiscoverySeedContribution"')
         cur.execute('DELETE FROM "ScanWorkItem"')
+        cur.execute('DELETE FROM "ApiCache"')
         cur.execute('DELETE FROM "Config"')
     connection.commit()
     connection.close()