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/tracks.sql.go

@@ -668,6 +668,7 @@ const listTracksMissingMbidWithPath = `-- name: ListTracksMissingMbidWithPath :m
 SELECT id, file_path
   FROM tracks
  WHERE mbid IS NULL
+   AND missing_since IS NULL
  ORDER BY id
  LIMIT $1
 `
@@ -679,6 +680,8 @@ type ListTracksMissingMbidWithPathRow struct {
 
 // Track recording-MBID backfill: tracks with NULL mbid that still have
 // a file to re-read. $1 caps the batch (mirrors the album backfill).
+// Missing tracks are skipped: there is no file, and opening it anyway
+// logged a warning per track on every scan (#5139).
 func (q *Queries) ListTracksMissingMbidWithPath(ctx context.Context, limit int32) ([]ListTracksMissingMbidWithPathRow, error) {
 	rows, err := q.db.Query(ctx, listTracksMissingMbidWithPath, limit)
 	if err != nil {