M1: key management, backup locations and occ commands

- migration: ngb_targets, ngb_runs, ngb_snapshots
- KeyService: master key stored encrypted with the instance secret
  (unattended backups), passphrase-wrapped copy in every repository and
  in the recovery kit, recorded admin confirmation per key fingerprint
- TargetService: locations built from files_external backends without a
  mount; files_external is loaded through IAppManager::loadApp() and its
  enabled state is left alone; credentials encrypted with ICrypto;
  existing repositories are adopted only if made with this key, and a
  repository cannot be registered twice
- Repository: initWithKey/openWithKey with an encrypted key check
- occ: backup:key:init|kit|confirm, backup:target:backends|add|list|remove

lib/Command/TargetBackends.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Command;
+
+use OC\Core\Command\Base;
+use OCA\NgBackup\Service\TargetService;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+class TargetBackends extends Base {
+	public function __construct(
+		private TargetService $targets,
+	) {
+		parent::__construct();
+	}
+
+	protected function configure(): void {
+		parent::configure();
+		$this->setName('backup:target:backends')
+			->setDescription('List the storage backends (from files_external) that can be used as backup location');
+	}
+
+	protected function execute(InputInterface $input, OutputInterface $output): int {
+		$backends = $this->targets->availableBackends();
+		if ($input->getOption('output') !== self::OUTPUT_FORMAT_PLAIN) {
+			$this->writeMixedInOutputFormat($input, $output, $backends);
+			return 0;
+		}
+		foreach ($backends as $b) {
+			$output->writeln(sprintf('<info>%s</info> (%s)  auth schemes: %s', $b['id'], $b['name'], implode(', ', $b['auth'])));
+			foreach ($b['parameters'] as $key => $p) {
+				$output->writeln(sprintf('    -o %s=...   %s', $key, is_array($p) ? ($p['value'] ?? '') : ''));
+			}
+		}
+		return 0;
+	}
+}