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>

tests/bootstrap.php

@@ -0,0 +1,5 @@
+<?php
+
+declare(strict_types=1);
+
+require_once __DIR__ . '/../vendor/autoload.php';