Add Deck workflow automation Nextcloud app

Ports the standalone due-date cron script into a proper Nextcloud 34
app with a personal-settings UI, per-user workflow configuration
(source/target stack, assigned-user and label filters, email
notification), a TimedJob background runner, and Gitea CI pipelines.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/lint-php.yml

@@ -0,0 +1,24 @@
+name: Lint PHP
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  php-lint:
+    runs-on: gitea-runner-server03
+    strategy:
+      matrix:
+        php-version: ['8.2', '8.3', '8.4']
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-version }}
+          coverage: none
+
+      - name: Syntax check
+        run: |
+          find lib tests appinfo -name '*.php' -print0 | xargs -0 -n1 -- php -l