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/missing_pairs.sql

@@ -25,6 +25,10 @@
 -- A pair whose MBIDs are both set and differ is not a title match: the tags say
 -- they are different recordings. The caller folds a pair only when each side has
 -- exactly one candidate; anything else is ambiguous and left alone.
+--
+-- A slot the duplicate resolver emptied (emptied_at, migration 0095) pairs by
+-- title only with a track added since: the album held no copy of the song as
+-- long as it is, so one already there under the title is another song.
 WITH grp AS (
   SELECT id AS album_id, COALESCE(release_group_mbid, id::text) AS g
     FROM albums
@@ -47,6 +51,7 @@ cand AS (
                  AND lower(p.title) = lower(m.title)
    WHERE m.missing_since IS NOT NULL
      AND (m.mbid IS NULL OR p.mbid IS NULL OR m.mbid = p.mbid)
+     AND (m.emptied_at IS DISTINCT FROM m.missing_since OR p.added_at > m.emptied_at)
 ),
 pairs AS (
   -- A pair both rules found is one candidate, credited to the stronger rule.