fix: a slot the resolver emptied folds only into a copy of its own song (M498)

On 2026-10-20 the emptied "Open the Gateway" slot of Auvic's Speed Demons
was folded by title into another file tagged "Open the Gateway" that holds
Onward (244 s against the song's 254 s), so the album was never wanted
back. A wrong-slot removal now stamps tracks.emptied_at (migration 0095)
when the album holds no copy of the song as long as its MusicBrainz slot,
and missing-pair folding pairs such a row by title only with a track added
since: a copy fetched again. A slot whose song the album does hold (VOID
DLC's Weeper beside Void's) folds into it as before.

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

internal/db/queries/tracks.sql

@@ -226,6 +226,15 @@ UPDATE tracks
    SET mbid = NULL, mbid_source = NULL
  WHERE id = ANY(sqlc.arg(ids)::uuid[]);
 
+-- name: MarkTracksEmptied :exec
+-- Stamps rows the duplicate resolver emptied, whose song the album holds no
+-- other copy of, with the time they went missing (migration 0095): missing-pair
+-- folding then waits for a copy fetched again.
+UPDATE tracks
+   SET emptied_at = missing_since
+ WHERE id = ANY(sqlc.arg(ids)::uuid[])
+   AND missing_since IS NOT NULL;
+
 -- 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