feat(library): undo auditor merges for 30 days, redirect old keys

V29 adds game_merge_undo and game_merge_undo_user: every new merge saves
a copy of what it changed. POST /api/library/merges/{mergeId}/undo
(auditors, newest merge first) puts it back; users keep later status
and rating, nothing is paid twice. A nightly job drops copies after 30
days. Old keys follow game_alias to the kept game. Manual games can now
be hidden and shown like Steam games.

src/main/resources/application.yml

@@ -65,6 +65,7 @@ app:
     edition-ttl-days: 30
     # Reported broken pictures, repaired after the nightly sync.
     image-repair-cron: "0 30 1 * * *"
+    merge-undo-purge-cron: "0 45 3 * * *"
     image-repair-batch: 200
     image-repair-max-attempts: 3
     image-repair-retry-days: 30