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>

lib/Service/DeckUnavailableException.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\WorkflowDeckAutomation\Service;
+
+/**
+ * Thrown whenever the Deck app is disabled for a user, or its internal
+ * classes could not be resolved. Deck's OCA\Deck\* classes are not a
+ * documented public API, so every access point is wrapped and normalised
+ * into this exception instead of leaking Deck-internal exception types.
+ */
+class DeckUnavailableException extends \RuntimeException {
+}