fix: a slot emptied of another song's file drops that file's recording MBID, so missing-pair folding cannot fold it into the copy kept (M498)

On 2026-10-20 twelve of the 24 slots the resolver emptied were folded by MBID
into the kept copy, so their albums were never wanted back.

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

internal/db/dbq/tracks.sql.go

@@ -56,6 +56,21 @@ func (q *Queries) ClearStaleSuspectSourceTracks(ctx context.Context, pattern str
 	return result.RowsAffected(), nil
 }
 
+const clearTrackMbids = `-- name: ClearTrackMbids :exec
+UPDATE tracks
+   SET mbid = NULL
+ WHERE id = ANY($1::uuid[])
+`
+
+// Forgets the recording MBID of rows whose file was another song's (M498's
+// wrong-slot removal). The MBID came from that file, so it names the song kept
+// elsewhere on the album, and missing-pair folding would fold the emptied slot
+// into it: twelve slots went that way on 2026-10-20 and were never wanted back.
+func (q *Queries) ClearTrackMbids(ctx context.Context, ids []pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, clearTrackMbids, ids)
+	return err
+}
+
 const clearTracksMissing = `-- name: ClearTracksMissing :execrows
 UPDATE tracks
    SET missing_since = NULL