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>

Makefile

@@ -0,0 +1,32 @@
+app_name=workflow_deck_automation
+build_directory=$(CURDIR)/build
+appstore_build_directory=$(build_directory)/appstore/$(app_name)
+appstore_artifact_directory=$(build_directory)/artifacts/appstore
+
+.PHONY: appstore
+appstore: clean
+	mkdir -p $(appstore_build_directory) $(appstore_artifact_directory)
+	rsync -av . $(appstore_build_directory) \
+		--exclude=/.git \
+		--exclude=/.gitea \
+		--exclude=/.gitignore \
+		--exclude=/composer.json \
+		--exclude=/composer.lock \
+		--exclude=/package.json \
+		--exclude=/package-lock.json \
+		--exclude=/node_modules \
+		--exclude=/vite.config.js \
+		--exclude=/src \
+		--exclude=/tests \
+		--exclude=/vendor \
+		--exclude=/build \
+		--exclude=/.php-cs-fixer.dist.php \
+		--exclude=/.php-cs-fixer.cache \
+		--exclude=/phpunit.xml \
+		--exclude=/README.md \
+		--exclude=/Makefile
+	tar -czf $(appstore_artifact_directory)/$(app_name).tar.gz -C $(build_directory)/appstore $(app_name)
+
+.PHONY: clean
+clean:
+	rm -rf $(build_directory)