Create archive task returns dict instead of string

This will save the task result in redis as a json object instead of a
json-encoded string. This makes for a nicer response from get_status and
prevents the client having to parse a json string to work with the
result.

src/worker.py

@@ -43,7 +43,7 @@ def create_archive_task(self, archive_json: str):
         logger.error(e)
         logger.error(traceback.format_exc())
         return {"error": e}
-    return result.to_json()
+    return result.to_dict()
 
 
 @celery.task(name="create_sheet_task", bind=True, autoretry_for=(Exception,), retry_backoff=True, retry_kwargs={'max_retries': 0})