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/queries/albums.sql

@@ -145,7 +145,9 @@ UPDATE albums
 -- name: ListAlbumsMissingMbidWithTrack :many
 -- 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).
 SELECT a.id          AS album_id,
        a.artist_id   AS artist_id,
        a.title       AS title,
@@ -155,6 +157,7 @@ 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

internal/db/queries/tracks.sql

@@ -26,9 +26,12 @@ RETURNING *;
 -- name: ListTracksMissingMbidWithPath :many
 -- 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).
 SELECT id, file_path
   FROM tracks
  WHERE mbid IS NULL
+   AND missing_since IS NULL
  ORDER BY id
  LIMIT $1;