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()

worker/tests/test_apicache.py

@@ -0,0 +1,104 @@
+"""Read-through + shared-shape tests for the worker cache. cached_api uses the real
+lyra_test DB (via the conn fixture); MB is never hit (fetch_fresh is a fake)."""
+from lyra_worker.apicache import cached_api, norm_key
+from lyra_worker._mbbrowser import (
+    _artist_hit_from_json, _artist_hit_to_json, _rg_from_json, _rg_to_json,
+)
+from lyra_worker.browser import ArtistHit, ReleaseGroupInfo
+
+
+def _backdate(conn, key, seconds):
+    with conn.cursor() as cur:
+        cur.execute(
+            'UPDATE "ApiCache" SET "fetchedAt" = now() - make_interval(secs => %s) WHERE key = %s',
+            (seconds, key),
+        )
+    conn.commit()
+
+
+def test_read_through_fetches_once(conn):
+    calls = []
+
+    def fresh():
+        calls.append(1)
+        return {"v": 1}
+
+    assert cached_api(conn, "k1", 3600, fresh) == {"v": 1}
+    assert cached_api(conn, "k1", 3600, fresh) == {"v": 1}
+    assert len(calls) == 1  # second served from cache
+
+
+def test_refetches_after_ttl(conn):
+    calls = []
+
+    def fresh():
+        calls.append(1)
+        return {"v": len(calls)}
+
+    cached_api(conn, "k2", 3600, fresh)
+    _backdate(conn, "k2", 7200)  # older than the ttl
+    cached_api(conn, "k2", 3600, fresh)
+    assert len(calls) == 2
+
+
+def test_serves_stale_on_fetch_error(conn):
+    cached_api(conn, "k3", 3600, lambda: {"v": "old"})
+    _backdate(conn, "k3", 7200)
+
+    def boom():
+        raise RuntimeError("mb down")
+
+    assert cached_api(conn, "k3", 3600, boom) == {"v": "old"}
+
+
+def test_reraises_when_no_cached_row(conn):
+    def boom():
+        raise RuntimeError("mb down")
+
+    try:
+        cached_api(conn, "k4-missing", 3600, boom)
+    except RuntimeError as e:
+        assert "mb down" in str(e)
+    else:
+        raise AssertionError("expected the fetch error to propagate")
+
+
+def test_does_not_cache_none(conn):
+    calls = []
+
+    def fresh():
+        calls.append(1)
+        return None
+
+    cached_api(conn, "k5", 3600, fresh)
+    cached_api(conn, "k5", 3600, fresh)
+    assert len(calls) == 2  # None never cached
+    with conn.cursor() as cur:
+        cur.execute('SELECT 1 FROM "ApiCache" WHERE key = %s', ("k5",))
+        assert cur.fetchone() is None
+
+
+def test_norm_key():
+    assert norm_key("  Radiohead ") == "radiohead"
+
+
+def test_release_group_json_roundtrip():
+    r = ReleaseGroupInfo(rg_mbid="rg1", title="OK Computer", primary_type="Album",
+                         secondary_types=("Live",), first_release_date="1997-05-21")
+    assert _rg_from_json(_rg_to_json(r)) == r
+    # a bare group (no type/date) round-trips to the worker's empty-string defaults
+    bare = ReleaseGroupInfo(rg_mbid="rg2", title="X")
+    assert _rg_from_json(_rg_to_json(bare)) == bare
+
+
+def test_artist_hit_json_roundtrip():
+    a = ArtistHit(mbid="a1", name="Radiohead", disambiguation="UK band")
+    assert _artist_hit_from_json(_artist_hit_to_json(a)) == a
+
+
+def test_worker_shape_matches_web_camelcase():
+    # the stored dict MUST use the same camelCase keys the web app writes/reads
+    d = _rg_to_json(ReleaseGroupInfo(rg_mbid="rg", title="T", primary_type="Album",
+                                     secondary_types=(), first_release_date="2000"))
+    assert set(d) == {"rgMbid", "title", "primaryType", "secondaryTypes", "firstReleaseDate"}
+    assert set(_artist_hit_to_json(ArtistHit(mbid="a", name="N"))) == {"mbid", "name", "disambiguation"}