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/mbidbackfill.go

@@ -167,3 +167,96 @@ func readMBIDsForFile(path string, logger *slog.Logger) (albumMBID, artistMBID s
 	}
 	return extractMBIDs(meta)
 }
+
+// BackfillTrackMBIDsResult tallies the track recording-MBID backfill.
+type BackfillTrackMBIDsResult struct {
+	Processed int // Tracks considered (mbid IS NULL).
+	Healed    int // Tracks whose recording MBID we set.
+	Skipped   int // Tracks whose file was missing / untagged / write failed.
+}
+
+// BackfillTrackMBIDs walks tracks with NULL mbid, re-reads each file's
+// tags, and persists the MusicBrainz recording MBID. This is the
+// counterpart to BackfillMBIDs (which heals album/artist MBIDs); it
+// unblocks the ListenBrainz similarity pipeline, which is gated on
+// tracks.mbid IS NOT NULL.
+//
+// Idempotent via SetTrackMbidIfNull: re-running costs the same N file
+// reads but is a no-op for already-healed rows. limit caps one call;
+// pass -1 for unbounded (caller normally batches per scan).
+func BackfillTrackMBIDs(ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger,
+	limit int, progressCb func(BackfillTrackMBIDsResult)) (BackfillTrackMBIDsResult, error) {
+	q := dbq.New(pool)
+	queryLimit := int32(limit)
+	if limit < 0 {
+		queryLimit = 1<<31 - 1 // effectively unbounded
+	}
+
+	rows, err := q.ListTracksMissingMbidWithPath(ctx, queryLimit)
+	if err != nil {
+		return BackfillTrackMBIDsResult{}, fmt.Errorf("list tracks missing mbid: %w", err)
+	}
+
+	var res BackfillTrackMBIDsResult
+	for i, r := range rows {
+		if ctx.Err() != nil {
+			return res, ctx.Err()
+		}
+		res.Processed++
+
+		rec := readRecordingMBIDForFile(r.FilePath, logger)
+		if rec == "" {
+			res.Skipped++
+			if progressCb != nil {
+				progressCb(res)
+			}
+			continue
+		}
+
+		m := rec
+		if uerr := q.SetTrackMbidIfNull(ctx, dbq.SetTrackMbidIfNullParams{
+			ID: r.ID, Mbid: &m,
+		}); uerr != nil {
+			logger.Warn("track mbid backfill: set failed",
+				"track_id", r.ID, "err", uerr)
+			res.Skipped++
+			if progressCb != nil {
+				progressCb(res)
+			}
+			continue
+		}
+
+		res.Healed++
+		if (i+1)%500 == 0 {
+			logger.Info("track mbid backfill progress",
+				"processed", res.Processed, "healed", res.Healed, "skipped", res.Skipped)
+		}
+		if progressCb != nil {
+			progressCb(res)
+		}
+	}
+
+	logger.Info("track mbid backfill complete",
+		"processed", res.Processed, "healed", res.Healed, "skipped", res.Skipped)
+	return res, nil
+}
+
+// readRecordingMBIDForFile opens one audio file and returns its
+// MusicBrainz recording MBID, or "" when the file can't be opened, the
+// tag read fails, or the tag is absent. Errors are logged at Warn, not
+// propagated — one broken file must not halt the backfill.
+func readRecordingMBIDForFile(path string, logger *slog.Logger) string {
+	f, err := os.Open(path)
+	if err != nil {
+		logger.Warn("track mbid backfill: open failed", "path", path, "err", err)
+		return ""
+	}
+	defer func() { _ = f.Close() }()
+
+	meta, err := tag.ReadFrom(f)
+	if err != nil {
+		logger.Warn("track mbid backfill: tag read failed", "path", path, "err", err)
+		return ""
+	}
+	return extractRecordingMBID(meta)
+}

internal/library/mbids.go

@@ -27,6 +27,19 @@ func extractMBIDs(m tag.Metadata) (albumMBID, artistMBID string) {
 	return cleanMBID(info.Get(mbz.Album)), cleanMBID(info.Get(mbz.Artist))
 }
 
+// extractRecordingMBID reads the MusicBrainz *recording* ID — Picard's
+// musicbrainz_recordingid, surfaced by dhowden/tag as mbz.Recording
+// (tag display name "MusicBrainz Track Id"). This is the id ListenBrainz
+// /explore/similar-recordings keys on and what tracks.mbid stores.
+//
+// Deliberately NOT mbz.Track ("MusicBrainz Release Track Id"): that is
+// per-release-track, whereas similarity is per-recording. Kept separate
+// from extractMBIDs so its existing (album, artist) signature and unit
+// tests stay untouched.
+func extractRecordingMBID(m tag.Metadata) string {
+	return cleanMBID(mbz.Extract(m).Get(mbz.Recording))
+}
+
 func cleanMBID(s string) string {
 	if s == "" {
 		return ""

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 {

internal/library/scanrun.go

@@ -154,6 +154,16 @@ func RunScan(
 		}
 	}
 
+	// Stage 2b: track recording-MBID backfill. Unblocks the ListenBrainz
+	// similarity pipeline (gated on tracks.mbid IS NOT NULL). Log-only
+	// progress — no scan_runs jsonb column to avoid a schema addition;
+	// it's idempotent and reruns each scan until every file is healed.
+	if cfg.BackfillCap != 0 {
+		if _, tberr := BackfillTrackMBIDs(ctx, pool, logger, cfg.BackfillCap, nil); tberr != nil {
+			captureErr("track_mbid_backfill", tberr)
+		}
+	}
+
 	// Stage 3: cover enrichment.
 	if enricher != nil && cfg.EnrichCap != 0 {
 		var lastP, lastS, lastF int