M5: notifications, setup check, audit log and password confirmation

- notifications to administrators for failed backup/restore runs and
  locations without a successful backup for two days (once a day)
- setup check in the admin overview: error when the key/kit is not set
  up, warning for missing, old or failed backups
- security events (recovery kit downloaded, location removed,
  append-only turned off, retention changed, snapshots forgotten) go to
  the audit log and are announced to all administrators, from the web
  and from occ; the scheduled weekly cleanup is logged only
- fix: the recovery-kit download link failed the CSRF check (412); it
  now requires a recent password confirmation instead, like creating the
  key, removing a location and restoring

lib/Command/TargetAppendOnly.php

@@ -16,6 +16,7 @@ use Symfony\Component\Console\Output\OutputInterface;
 class TargetAppendOnly extends Command {
 	public function __construct(
 		private TargetService $targets,
+		private \OCA\NgBackup\Service\AlertService $alerts,
 	) {
 		parent::__construct();
 	}
@@ -35,6 +36,9 @@ class TargetAppendOnly extends Command {
 			return 1;
 		}
 		$t = $this->targets->setAppendOnly($this->targets->get($input->getArgument('target')), $state === 'on');
+		if ($state === 'off') {
+			$this->alerts->securityEvent('append_only_off', ['target' => $t->getName()]);
+		}
 		$output->writeln($t->getName() . ' is ' . ($t->getAppendOnly() ? 'append-only' : 'normal') . '.');
 		return 0;
 	}