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/Command/KeySlotAdd.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Command;
+
+use OCA\NgBackup\Service\KeyService;
+use OCA\NgBackup\Service\KeySlotService;
+
+class KeySlotAdd extends KeySlots {
+	public function __construct(KeyService $keys, KeySlotService $slots) {
+		parent::__construct($keys, $slots, 'add');
+	}
+}