fix(library): extract recording MBID → tracks.mbid; unblock LB similarity

tracks.mbid was 100% NULL: the scanner only extracted album + artist
MBIDs, never the recording MBID. The ListenBrainz similarity worker is
gated on tracks.mbid IS NOT NULL, so track_similarity could never
populate — starving For-You/radio's strongest candidate source
(lb_similar) on every library.

- mbids.go: add extractRecordingMBID (mbz.Recording / Picard
  musicbrainz_recordingid). Separate fn so extractMBIDs' signature +
  unit tests stay untouched.
- scanner.go: persist recording MBID via UpsertTrack (heals on the
  file_path conflict, so re-scans backfill for free).
- BackfillTrackMBIDs: one-shot pass mirroring BackfillMBIDs, wired as
  scan Stage 2b (idempotent via SetTrackMbidIfNull, gated by
  BackfillCap, log-only progress).
- migration 0029: tracks_mbid_unique (0002, written when the column
  was always NULL) wrongly assumes one MBID == one track. A recording
  appears on multiple releases, so rows legitimately share a recording
  MBID. Replace with a non-unique partial index. Zero-risk: column is
  100% NULL at migration time.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/library/scanner.go

@@ -142,6 +142,7 @@ func (s *Scanner) scanFile(ctx context.Context, q *dbq.Queries, path string, sta
 		return fmt.Errorf("tag read: %w", err)
 	}
 	albumMBID, artistMBID := extractMBIDs(meta)
+	recordingMBID := extractRecordingMBID(meta)
 
 	artistName := meta.Artist()
 	if artistName == "" {
@@ -195,6 +196,13 @@ func (s *Scanner) scanFile(ctx context.Context, q *dbq.Queries, path string, sta
 	if g := meta.Genre(); g != "" {
 		params.Genre = &g
 	}
+	// Recording MBID feeds the ListenBrainz similarity pipeline.
+	// UpsertTrack heals mbid on the file_path conflict, so a re-scan
+	// of a previously-untagged-into-DB track backfills it for free.
+	if recordingMBID != "" {
+		m := recordingMBID
+		params.Mbid = &m
+	}
 
 	track, err := q.UpsertTrack(ctx, params)
 	if err != nil {