M2: full backup runs (database, config, data) with occ and cron

BackupService starts a run per location (database dump in one
transaction, then config + data directory as a resumable file run),
stores the state in ngb_runs after every step, locks a run while a step
executes and records the snapshot. Previews, logs, updater backups and
per-user cache/uploads are excluded (TreeWalker now supports patterns).
BackupJob (every 5 min) starts the daily schedule and steps running runs
with a short budget under web cron and a long one under system cron.
occ: backup:run, backup:status, backup:list.

Tested on NC34/PostgreSQL: first run 128 files / 1 GiB in 64 s (443 MiB
uploaded to S3), unchanged second run 2 s / 0 bytes; a background run to
SFTP completed through cron.php.

lib/Db/SnapshotMapper.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Db;
+
+use OCP\AppFramework\Db\QBMapper;
+use OCP\DB\QueryBuilder\IQueryBuilder;
+use OCP\IDBConnection;
+
+/** @template-extends QBMapper<Snapshot> */
+class SnapshotMapper extends QBMapper {
+	public function __construct(IDBConnection $db) {
+		parent::__construct($db, 'ngb_snapshots', Snapshot::class);
+	}
+
+	/** @return list<Snapshot> newest first */
+	public function findForTarget(int $targetId): array {
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')->from($this->getTableName())
+			->where($qb->expr()->eq('target_id', $qb->createNamedParameter($targetId, IQueryBuilder::PARAM_INT)))
+			->orderBy('created_at', 'DESC');
+		return $this->findEntities($qb);
+	}
+
+	public function findOne(int $targetId, string $snapshotId): Snapshot {
+		$qb = $this->db->getQueryBuilder();
+		$qb->select('*')->from($this->getTableName())
+			->where($qb->expr()->eq('target_id', $qb->createNamedParameter($targetId, IQueryBuilder::PARAM_INT)))
+			->andWhere($qb->expr()->eq('snapshot_id', $qb->createNamedParameter($snapshotId)));
+		return $this->findEntity($qb);
+	}
+
+	public function latest(int $targetId, string $kind): ?Snapshot {
+		foreach ($this->findForTarget($targetId) as $s) {
+			if ($s->getKind() === $kind) {
+				return $s;
+			}
+		}
+		return null;
+	}
+}