fix(library): MBID backfills skip tracks whose files are missing (#5139)

The track backfill listed every track with a NULL mbid, missing or
not, so each scan tried to open every missing file and logged an
"open failed" warning per track. Nothing ever healed. The album
backfill could pick a missing track as the one to read, and since
that pass is capped per scan, albums stuck that way were retried
ahead of the rest every time.

Both now read only tracks still on disk; an album with none left is
skipped until a scan finds its files again.

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

internal/db/dbq/albums.sql.go

@@ -552,6 +552,7 @@ SELECT a.id          AS album_id,
     SELECT file_path
       FROM tracks
      WHERE album_id = a.id
+       AND missing_since IS NULL
      ORDER BY disc_number NULLS LAST, track_number NULLS LAST, id
      LIMIT 1
   ) t ON true
@@ -569,7 +570,9 @@ type ListAlbumsMissingMbidWithTrackRow struct {
 
 // One-shot MBID backfill: returns each album where mbid IS NULL alongside
 // one of its tracks' file_path so the worker can re-read tags. LIMIT
-// supplied by caller for batching/progress purposes.
+// supplied by caller for batching/progress purposes. The track read is
+// one still on disk; an album with none left is skipped until a scan
+// finds its files again (#5139).
 func (q *Queries) ListAlbumsMissingMbidWithTrack(ctx context.Context, limit int32) ([]ListAlbumsMissingMbidWithTrackRow, error) {
 	rows, err := q.db.Query(ctx, listAlbumsMissingMbidWithTrack, limit)
 	if err != nil {