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-info-xml.yml

@@ -0,0 +1,23 @@
+name: Lint info.xml
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  xml-lint:
+    runs-on: gitea-runner-server03
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install xmllint
+        run: |
+          apt-get update
+          apt-get install -y --no-install-recommends libxml2-utils
+
+      - name: Download appstore schema
+        run: wget -q https://raw.githubusercontent.com/nextcloud/appstore/master/nextcloudappstore/api/v1/release/info.xsd -O info.xsd
+
+      - name: Validate appinfo/info.xml
+        run: xmllint --schema info.xsd appinfo/info.xml --noout