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/queries/tracks.sql

@@ -216,6 +216,15 @@ UPDATE tracks
  WHERE id = ANY(sqlc.arg(ids)::uuid[])
    AND missing_since IS NULL;
 
+-- name: ClearTrackMbids :exec
+-- 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.
+UPDATE tracks
+   SET mbid = NULL
+ WHERE id = ANY(sqlc.arg(ids)::uuid[]);
+
 -- name: ClearTracksMissing :execrows
 -- Clears the mark on rows whose file is back. Runs independently of the mtime
 -- skip check, so a file that reappears unchanged is un-marked even though the