refactor code dir

rename backend/ to api/, remove leftover dir ui/

api/celery_app.py

@@ -0,0 +1,9 @@
+from celery import Celery
+
+app = Celery('backend')
+
+# Load configuration from Django settings, using the CELERY namespace.
+app.config_from_object('django.conf:settings', namespace='CELERY')
+
+# Autodiscover tasks from installed apps.
+app.autodiscover_tasks()