M4: admin settings page and JSON API

- AdminApiController (admin only, CSRF-protected): status, create key,
  download recovery kit, confirm (statement accepted + exact phrase),
  backends with their usable auth mechanisms, add/test/remove locations,
  start backups, schedule, snapshots, browse, queue restores
- RestoreJob runs web-initiated restores in the background; backup and
  restore runs are kept apart in the run table
- admin section 'NG Backup' with a Vue 3 / @nextcloud/vue 9 page: key and
  recovery-kit flow (download, tick statement, 'Yes, I confirm'), location
  form built from files_external's parameter definitions, schedule, run
  list with polling, snapshot browser and restore with the three modes
- auth mechanisms that depend on a user session or on files_external's
  own stored credentials are not offered

API tested with curl on NC34 (status, backends, schedule validation,
confirmation refusal, browse, queued restore and backup completed by
cron); non-admins get 403, anonymous 401.

img/app.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="24" height="24"><path fill="#fff" d="M12 2 4 5v6c0 5 3.4 9.7 8 11 4.6-1.3 8-6 8-11V5l-8-3Zm0 5a3 3 0 0 1 3 3v1h1v6H8v-6h1v-1a3 3 0 0 1 3-3Zm0 1.6c-.8 0-1.4.6-1.4 1.4v1h2.8v-1c0-.8-.6-1.4-1.4-1.4Z"/></svg>