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/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 ""