M5: deletion delay with trash, passphrase slots, ransomware documentation

- removed snapshots go to a trash on the location and their data is only
  freed after the deletion delay, which is chosen when the key is created
  (default 7 days, 1-365) and cannot be changed afterwards; any admin can
  bring them back (occ backup:trash, backup:trash:restore, web UI)
- up to three passphrase slots open the same master key (like LUKS), so
  one person leaving or dying does not lock the company out; slot changes
  are written to every location, announced to all admins and require a
  new recovery kit confirmation, while backups keep running
- occ backup:key:slots, backup:key:slot:add|replace|remove; key init takes
  --label and --delete-delay-days
- web UI: deletion delay and first slot at key creation, slot management,
  trash per location with restore; all behind password confirmation
- README: key, slots (safe / CTO / head of IT), and protection against
  ransomware: object lock in compliance mode (e.g. 90 days), credentials
  without delete rights, provider snapshots, off-site copy

lib/Service/AlertService.php

@@ -35,8 +35,12 @@ class AlertService {
 	 *
 	 * @param array<string, string> $params
 	 */
+	public function actor(): string {
+		return $this->session->getUser()?->getUID() ?? (PHP_SAPI === 'cli' ? 'occ' : 'unknown');
+	}
+
 	public function securityEvent(string $event, array $params = [], bool $notify = true): void {
-		$actor = $this->session->getUser()?->getUID() ?? (PHP_SAPI === 'cli' ? 'occ' : 'unknown');
+		$actor = $this->actor();
 		$params = ['actor' => $actor] + $params;
 		$this->events->dispatchTyped(new \OCP\Log\Audit\CriticalActionPerformedEvent(
 			'NG Backup: %s by %s ' . json_encode(array_diff_key($params, ['actor' => 1]), JSON_UNESCAPED_SLASHES), ['event' => $event, 'actor' => $actor]));