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/lyra_worker/registry.py

@@ -1,4 +1,4 @@
-from lyra_worker._mbbrowser import MusicBrainzBrowser
+from lyra_worker._mbbrowser import CachingMbBrowser, MusicBrainzBrowser
 from lyra_worker._musicbrainz import MusicBrainzResolver
 from lyra_worker._mutagen import MutagenTagger
 from lyra_worker._probe import MutagenProbe
@@ -42,9 +42,12 @@ def build_tagger() -> Tagger:
     return MutagenTagger()
 
 
-def build_browser() -> MbBrowser:
-    """The MusicBrainz browser used by the background monitor."""
-    return MusicBrainzBrowser()
+def build_browser(dsn: str | None = None) -> MbBrowser:
+    """The MusicBrainz browser used by the background monitor. When a DSN is given, wrap it
+    in the shared ApiCache read-through so scans/discovery stop re-fetching discographies
+    (and warm the same cache the web modals read)."""
+    inner = MusicBrainzBrowser()
+    return CachingMbBrowser(inner, dsn) if dsn else inner
 
 
 def build_probe() -> AudioProbe:
@@ -52,14 +55,15 @@ def build_probe() -> AudioProbe:
     return MutagenProbe()
 
 
-def build_similarity_sources(config: dict) -> list[SimilaritySource]:
+def build_similarity_sources(config: dict, dsn: str | None = None) -> list[SimilaritySource]:
     """Similarity sources for discovery. ListenBrainz needs no credentials, so it is
     always constructed; per-run reachability is checked via each source's health().
-    Last.fm is added only when an API key is configured."""
+    Last.fm is added only when an API key is configured. Its MB name→MBID lookups ride the
+    shared ApiCache when a DSN is given."""
     sources: list[SimilaritySource] = [
         ListenBrainzSource(base_url=config.get("discover.listenBrainzUrl") or "")
     ]
     key = config.get("lastfm.api_key")
     if key:
-        sources.append(LastfmSource(api_key=key, browser=MusicBrainzBrowser()))
+        sources.append(LastfmSource(api_key=key, browser=build_browser(dsn)))
     return sources