use celery for asynchronous tasks, rename session_uuid -> file_uuid

api/backend/celery.py

@@ -0,0 +1,9 @@
+import os
+from celery import Celery
+
+os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'backend.settings')
+
+app = Celery('backend')
+# Load config from Django settings, using a `CELERY_` prefix
+app.config_from_object('django.conf:settings', namespace='CELERY')
+app.autodiscover_tasks()