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.

lib/Command/TargetBackends.php

@@ -32,10 +32,16 @@ class TargetBackends extends Base {
 			return 0;
 		}
 		foreach ($backends as $b) {
-			$output->writeln(sprintf('<info>%s</info> (%s)  auth schemes: %s', $b['id'], $b['name'], implode(', ', $b['auth'])));
+			$output->writeln(sprintf('<info>%s</info> (%s)', $b['id'], $b['name']));
 			foreach ($b['parameters'] as $key => $p) {
 				$output->writeln(sprintf('    -o %s=...   %s', $key, is_array($p) ? ($p['value'] ?? '') : ''));
 			}
+			foreach ($b['auth'] as $a) {
+				$output->writeln(sprintf('  -a %s  (%s)', $a['id'], $a['name']));
+				foreach ($a['parameters'] as $key => $p) {
+					$output->writeln(sprintf('      -o %s=...   %s', $key, is_array($p) ? ($p['value'] ?? '') : ''));
+				}
+			}
 		}
 		return 0;
 	}