Add MusicBrainz search-by-name enrichment for scrobbles without IDs

When a listen arrives with no MBZ IDs and no album title (the common
multi-scrobbler/Last.fm case), search MusicBrainz by artist+track name
to resolve recording, release, and release group IDs. This unlocks
CoverArtArchive album art, proper album association, and duration data.

New file: internal/mbz/search.go
- SearchRecording() method with Lucene query escaping
- Confidence filter: case-insensitive exact match on title + artist credit
- Release selection: prefer Official status, then first available
- Uses existing rate-limited queue (1 req/sec)

Integration in catalog.go:
- Only triggers when RecordingMbzID, ReleaseMbzID, AND ReleaseTitle are
  all missing — no impact on scrobbles that already have MBZ data
- Soft failure — search errors don't block the listen
- KOITO_DISABLE_MUSICBRAINZ handled automatically (MbzErrorCaller returns error)

Interface + mocks updated:
- SearchRecording added to MusicBrainzCaller interface
- MbzMockCaller: SearchResults map for test data
- MbzErrorCaller: returns error (existing pattern)

New tests:
- TestSubmitListen_SearchByName — mock search, verify album+duration resolved
- TestSubmitListen_SearchByNameNoMatch — verify graceful fallback

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/mbz/mock.go

@@ -15,6 +15,7 @@ type MbzMockCaller struct {
 	ReleaseGroups map[uuid.UUID]*MusicBrainzReleaseGroup
 	Releases      map[uuid.UUID]*MusicBrainzRelease
 	Tracks        map[uuid.UUID]*MusicBrainzTrack
+	SearchResults map[string]*MusicBrainzSearchResult
 }
 
 func (m *MbzMockCaller) GetReleaseGroup(ctx context.Context, id uuid.UUID) (*MusicBrainzReleaseGroup, error) {
@@ -70,6 +71,14 @@ func (m *MbzMockCaller) GetArtistPrimaryAliases(ctx context.Context, id uuid.UUI
 	return ss, nil
 }
 
+func (m *MbzMockCaller) SearchRecording(ctx context.Context, artist string, track string) (*MusicBrainzSearchResult, error) {
+	key := artist + "\x00" + track
+	if result, exists := m.SearchResults[key]; exists {
+		return result, nil
+	}
+	return nil, nil
+}
+
 func (m *MbzMockCaller) Shutdown() {}
 
 type MbzErrorCaller struct{}
@@ -94,4 +103,8 @@ func (m *MbzErrorCaller) GetArtistPrimaryAliases(ctx context.Context, id uuid.UU
 	return nil, fmt.Errorf("error: GetArtistPrimaryAliases not implemented")
 }
 
+func (m *MbzErrorCaller) SearchRecording(ctx context.Context, artist string, track string) (*MusicBrainzSearchResult, error) {
+	return nil, fmt.Errorf("error: SearchRecording not implemented")
+}
+
 func (m *MbzErrorCaller) Shutdown() {}