Fold only sidecar-only releases (sidecar_only at insert); no queue-wide scan for gone releases

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

tests/functional/par2_merger.spec.ts

@@ -220,6 +220,8 @@ test.group('Par2Merger', (group) => {
     // A picture isn't a sidecar; an nfo an hour later belongs to nothing here.
     const picture = await release('cover', 1, null, at, ['cover.jpg'])
     const late = await release('Murdoch Mysteries S8D1', 1, null, new Date(at.getTime() + 3600_000), ['Murdoch Mysteries S8D1.nfo'])
+    // Queued, its release gone meanwhile (merged): leaves the queue with its batch.
+    await db.table('fold_candidates').insert({ release_id: 987654321 })
 
     const round = await new Par2Merger().round()
     assert.deepEqual(round.folded, { looked: 5, sidecars: 2, folded: 1 })