More reliable background workers

At the moment queue listener starts in FastApi background task https://fastapi.tiangolo.com/tutorial/background-tasks/
It's not very reliable for long running tasks and we have not ability to control tasks status/count.
Probably should look into variants like celery.

Start by reviewing the linked FastAPI background-tasks documentation and the current queue listener entry point. Define how long-running tasks should be managed and how their status and count should be exposed, then evaluate whether a worker system such as Celery meets those requirements.

Written by the indexing model from the issue text.