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/build-release.yml

@@ -0,0 +1,35 @@
+name: Build release artifact
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+jobs:
+  package:
+    runs-on: gitea-runner-server03
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '24'
+
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.3'
+          coverage: none
+
+      - name: Install JS dependencies
+        run: npm ci
+
+      - name: Build frontend
+        run: npm run build
+
+      - name: Package appstore artifact
+        run: make appstore
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: workflow_deck_automation-${{ gitea.ref_name }}
+          path: build/artifacts/appstore/*.tar.gz