M5: end-to-end test matrix and CI

- tests/integration/roundtrip.sh: key, kit code and confirmation,
  location, full and unchanged incremental backup, restore new-folder and
  replace (files identical, extra file in the trash), database dump and
  restore, retention, trash and restore from trash, no files_external
  errors in the log
- tests/integration/matrix.sh: fresh Nextcloud per version and database
  (SQLite, MariaDB, PostgreSQL), removes only its own containers and
  their volumes; locally green on NC32 and NC35 with all three databases
- GitHub Actions: unit tests on PHP 8.2-8.4, round trip on NC32-35 x
  SQLite/MariaDB/PostgreSQL

Fixes found by the matrix:
- files_external is no longer booted when it is disabled: only its
  classes are loaded and a private BackendService gets its backends from
  its Application class (booting registered its mount provider, which
  failed on installations where its tables were never created)
- SQLite: the database file is excluded from the file backup (it is
  covered by the logical dump and copying the live file is inconsistent)
- occ backup:key:confirm accepts --code and --confirm for scripts

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+# SPDX-FileCopyrightText: 2026 Dillard Blom
+# SPDX-License-Identifier: AGPL-3.0-or-later
+name: CI
+
+on:
+  push:
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  unit:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php: ['8.2', '8.3', '8.4']
+    steps:
+      - uses: actions/checkout@v4
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          extensions: sodium, zip
+          coverage: none
+      - run: composer install --no-interaction --no-progress
+      - run: vendor/bin/phpunit -c tests/phpunit.xml
+
+  integration:
+    # Full round trip (key, kit, location, backup, incremental, restore modes, database dump and
+    # restore, retention, trash) on a fresh Nextcloud per version and database.
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        nextcloud: ['32', '33', '34', '35']
+        database: ['sqlite', 'mariadb', 'pgsql']
+    steps:
+      - uses: actions/checkout@v4
+      - name: Round trip on Nextcloud ${{ matrix.nextcloud }} / ${{ matrix.database }}
+        run: tests/integration/matrix.sh "${{ matrix.nextcloud }}" "${{ matrix.database }}"