Backups to SMB no longer crawl; add occ backup:cancel

Each 30-second step opened the location again and read its list of
index files. Over SMB that cost a request per file, so on a large
repository opening took longer than the step: each step stored one
4 MiB pack and the list grew further. A first backup of nc01 to a
Storage Box over SMB managed 12 GiB in 35 hours.

occ backup:run and system cron now keep the repository open for the
whole run (reopened when another process took a step or the location
was edited), opening no longer counts towards the step's time, and
SMB folders are listed in one request.

occ backup:cancel stops a running backup: no new step starts and the
step in progress finishes first. A run of an earlier version, which
does not hold back its next step, is marked cancelled after two
minutes unless it is recording its snapshot.

lib/Command/CancelRun.php

@@ -0,0 +1,74 @@
+<?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\Db\Run;
+use OCA\NgBackup\Db\RunMapper;
+use OCA\NgBackup\Service\BackupService;
+use OCP\AppFramework\Db\DoesNotExistException;
+use Symfony\Component\Console\Command\Command;
+use Symfony\Component\Console\Input\InputArgument;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+final class CancelRun extends Command {
+	public function __construct(
+		private RunMapper $runs,
+		private BackupService $backups,
+	) {
+		parent::__construct();
+	}
+
+	protected function configure(): void {
+		$this->setName('backup:cancel')
+			->setDescription('Stop a running backup')
+			->addArgument('run', InputArgument::REQUIRED, 'Run number, as shown by backup:status')
+			->setHelp("No new step of the run starts; the step in progress finishes first (usually within a
+"
+				. "minute), so a backup that is just being completed is not cut off. The process working on the
+"
+				. "run (occ backup:run, or cron) then stops. A backup started by an earlier version is marked
+"
+				. "cancelled after two minutes and stops at the end of its step. What it already stored is
+"
+				. "removed by the next cleanup.");
+	}
+
+	protected function execute(InputInterface $input, OutputInterface $output): int {
+		$id = (string)$input->getArgument('run');
+		if (!ctype_digit($id)) {
+			$output->writeln('<error>Give the run number, as shown by backup:status.</error>');
+			return 1;
+		}
+		try {
+			$run = $this->runs->find((int)$id);
+		} catch (DoesNotExistException) {
+			$output->writeln("<error>There is no run $id.</error>");
+			return 1;
+		}
+		if ($run->getKind() !== BackupService::KIND_FULL) {
+			$output->writeln("<error>Run $id is not a backup; only backups can be cancelled.</error>");
+			return 1;
+		}
+		if ($run->getStatus() === Run::RUNNING) {
+			$output->writeln("Waiting for the current step of run $id to finish...");
+		}
+		try {
+			$run = $this->backups->cancel($run);
+		} catch (\RuntimeException $e) {
+			$output->writeln('<error>' . $e->getMessage() . '</error>');
+			return 1;
+		}
+		if ($run->getStatus() !== Run::CANCELLED) {
+			$output->writeln("Run $id is not running (status: {$run->getStatus()}); nothing to cancel.");
+			return 0;
+		}
+		$output->writeln("Run $id is cancelled.");
+		return 0;
+	}
+}