M2: local cache of pack indexes (ngb_index)

Opening a repository lists index/ once and downloads only index files
that are not cached; packs written by this server are cached on write,
stale cache rows are dropped. Each resumable step therefore no longer
downloads every index file from the location.

lib/Repository/IIndexCache.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Repository;
+
+/** Optional local cache of pack index entries (the repository's index/ files stay authoritative). */
+interface IIndexCache {
+	/** @return array<string, list<array{id:string, offset:int, length:int, raw:int, flags:int}>> packId => entries */
+	public function all(string $repositoryId): array;
+
+	/** @param list<array{id:string, offset:int, length:int, raw:int, flags:int}> $entries */
+	public function put(string $repositoryId, string $packId, array $entries): void;
+
+	/** @param list<string> $packIds */
+	public function remove(string $repositoryId, array $packIds): void;
+}