Review fixes: a missed passphrase change follows at the next backup, a key from before 1.1.0 counts as used, schedules edited in their own time zone

- A location that a passphrase change could not reach gets it at its next backup
- A key from before 1.1.0 is never replaced as an unused one
- An unreadable settings file on a location is told apart from a missing one
- A schedule is shown and saved in the zone it was saved in
- Saving an unchanged schedule keeps its moment; --timezone needs a schedule
- Schedule coverage uses the time the backup began reading its data
- Tests: catch-up scenario, upgrade test restores the new file, autumn clock change

lib/Migration/Version000009Date20261108000000.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Migration;
+
+use Closure;
+use OCA\NgBackup\Service\KeyService;
+use OCP\Migration\IOutput;
+use OCP\Migration\SimpleMigrationStep;
+
+/**
+ * A key from before 1.1.0 counts as used: those versions did not keep track, and it may protect
+ * backups on a location that was removed from this server later (user exports included). Only a
+ * key made with 1.1.0 or newer can be replaced as an unused one.
+ */
+final class Version000009Date20261108000000 extends SimpleMigrationStep {
+	public function __construct(
+		private KeyService $keys,
+	) {
+	}
+
+	public function postSchemaChange(IOutput $output, Closure $schemaClosure, array $options): void {
+		if ($this->keys->isInitialized()) {
+			$this->keys->markUsed();
+		}
+	}
+}