Phase 0: skeleton, streaming encryption (spike A)

Repository skeleton (app id ng_backup, AGPL-3.0-or-later), KeyRing with
KDF subkeys, keyed blob ids and Argon2id key wrapping, and StreamCipher
(secretstream XChaCha20-Poly1305, framed, bounded memory).

Spike A: 1 GiB round trip with memory_limit=64M, peak < 2 MiB, ~160/122
MiB/s; tampering, truncation, wrong key/passphrase are rejected.

lib/AppInfo/Application.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\AppInfo;
+
+use OCP\AppFramework\App;
+use OCP\AppFramework\Bootstrap\IBootContext;
+use OCP\AppFramework\Bootstrap\IBootstrap;
+use OCP\AppFramework\Bootstrap\IRegistrationContext;
+
+class Application extends App implements IBootstrap {
+	public const APP_ID = 'ng_backup';
+
+	public function __construct() {
+		parent::__construct(self::APP_ID);
+	}
+
+	public function register(IRegistrationContext $context): void {
+	}
+
+	public function boot(IBootContext $context): void {
+	}
+}