feat(library): store each album's MusicBrainz release-group id (M483 #5242)

albums.mbid is the release id (Picard's musicbrainz_albumid, one edition).
Lidarr names albums by release group, so re-acquisition and request
completion need that id too (#5241).

Migration 0070 adds albums.release_group_mbid (nullable, non-unique index:
several releases share a group). The scanner reads musicbrainz_releasegroupid
through extractReleaseGroupMBID, writes it on insert and heals it onto
existing rows when NULL. tagReadVersion goes to 3 so the next scan fills
it for the library already indexed, bound by tag reads (no ffprobe, no
decode).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/scanner.go

@@ -56,7 +56,11 @@ var audioExtensions = map[string]bool{
 //	   "EDM", "Children'S Music" -> "Children's Music". Bumped rather than left
 //	   to new files only because taste_profile.sql reads tracks.genre directly,
 //	   so a half-repaired library would carry both spellings as separate tags.
-const tagReadVersion int16 = 2
+//	3: the MusicBrainz release-group id is read into albums.release_group_mbid
+//	   (#5241). Lidarr names albums by release group, not by the release id
+//	   albums.mbid holds, so re-acquisition and request completion need it for
+//	   the albums already in the library, not only for files added from now on.
+const tagReadVersion int16 = 3
 
 type Stats struct {
 	Scanned int `json:"scanned"`
@@ -258,6 +262,7 @@ func (s *Scanner) scanFile(
 	}
 	albumMBID, artistMBID := extractMBIDs(meta)
 	recordingMBID := extractRecordingMBID(meta)
+	releaseGroupMBID := extractReleaseGroupMBID(meta)
 
 	artistName := meta.Artist()
 	if artistName == "" {
@@ -276,7 +281,7 @@ func (s *Scanner) scanFile(
 	if err != nil {
 		return pgtype.UUID{}, false, fmt.Errorf("artist: %w", err)
 	}
-	album, err := s.resolveAlbum(ctx, q, artist.ID, albumTitle, meta.Year(), albumMBID)
+	album, err := s.resolveAlbum(ctx, q, artist.ID, albumTitle, meta.Year(), albumMBID, releaseGroupMBID)
 	if err != nil {
 		return pgtype.UUID{}, false, fmt.Errorf("album: %w", err)
 	}
@@ -518,9 +523,23 @@ func (s *Scanner) resolveArtist(ctx context.Context, q *dbq.Queries, name, mbid
 	return artist, nil
 }
 
-func (s *Scanner) resolveAlbum(ctx context.Context, q *dbq.Queries, artistID pgtype.UUID, title string, year int, mbid string) (dbq.Album, error) {
+func (s *Scanner) resolveAlbum(ctx context.Context, q *dbq.Queries, artistID pgtype.UUID, title string, year int, mbid, releaseGroupMBID string) (dbq.Album, error) {
 	existing, err := q.GetAlbumByArtistAndTitle(ctx, dbq.GetAlbumByArtistAndTitleParams{ArtistID: artistID, Title: title})
 	if err == nil {
+		// Heal the release group the same way. Not unique, so no conflict to
+		// handle: every release of a group may carry it.
+		if releaseGroupMBID != "" && existing.ReleaseGroupMbid == nil {
+			rg := releaseGroupMBID
+			if uerr := q.SetAlbumReleaseGroupMbidIfNull(ctx, dbq.SetAlbumReleaseGroupMbidIfNullParams{
+				ID:               existing.ID,
+				ReleaseGroupMbid: &rg,
+			}); uerr != nil {
+				s.logger.Warn("library scan: heal album release group failed",
+					"album_id", existing.ID, "err", uerr)
+			} else {
+				existing.ReleaseGroupMbid = &rg
+			}
+		}
 		// Heal: backfill mbid on a previously-imported row if we have one now.
 		if mbid != "" && (existing.Mbid == nil || *existing.Mbid == "") {
 			m := mbid
@@ -564,6 +583,10 @@ func (s *Scanner) resolveAlbum(ctx context.Context, q *dbq.Queries, artistID pgt
 		m := mbid
 		params.Mbid = &m
 	}
+	if releaseGroupMBID != "" {
+		rg := releaseGroupMBID
+		params.ReleaseGroupMbid = &rg
+	}
 	album, err := q.UpsertAlbum(ctx, params)
 	if err != nil {
 		return dbq.Album{}, err