fix: a merge keeps the song link when the removed copy names the song (M498)

MergeInheritSongLink took "linked" to mean song_key <> id, but the copy
whose id became the song's key has no song_id of its own. Removing that
copy dropped the survivor out of the song. It now asks whether another
copy shares the key. The test covers both copies as the one removed, so
it no longer passes or fails on which random id sorts first.

The release-change test counted audit rows other tests left behind
(ResetDB keeps audit_log); it now counts what its own pass adds.

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

internal/db/queries/songs.sql

@@ -48,11 +48,13 @@ SELECT id, song_key, (source_verdict IS NOT DISTINCT FROM 'suspect')::boolean AS
 -- name: MergeInheritSongLink :exec
 -- A merge keeps the removed copy's song link: when the copy being removed was
 -- linked to copies on other releases and the survivor was not, the survivor
--- joins that song.
+-- joins that song. The copy whose id names the song has no song_id of its
+-- own, so "linked" is another copy sharing its key, not a song_id.
 UPDATE tracks s
    SET song_id = l.song_key
   FROM tracks l
  WHERE s.id = sqlc.arg(survivor_id)::uuid
    AND l.id = sqlc.arg(loser_id)::uuid
    AND s.song_id IS NULL
-   AND l.song_key <> l.id;
+   AND EXISTS (SELECT 1 FROM tracks o
+                WHERE o.song_key = l.song_key AND o.id <> l.id AND o.id <> s.id);