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.

appinfo/info.xml

@@ -5,7 +5,7 @@
     <name>NG Backup</name>
     <summary>Encrypted, incremental backups of your Nextcloud with restore from the web interface</summary>
     <description><![CDATA[Work in progress, not ready for use.]]></description>
-    <version>0.0.4</version>
+    <version>0.0.5</version>
     <licence>agpl</licence>
     <author>Dillard Blom</author>
     <namespace>NgBackup</namespace>
@@ -33,4 +33,8 @@
         <command>OCA\NgBackup\Command\Browse</command>
         <command>OCA\NgBackup\Command\Restore</command>
     </commands>
+    <settings>
+        <admin>OCA\NgBackup\Settings\Admin</admin>
+        <admin-section>OCA\NgBackup\Settings\AdminSection</admin-section>
+    </settings>
 </info>