Please fix conflicts @Yannicked

* I think you need to make `celery` a `server` dependency in `pyproject.toml` (or guard the import of `celery`) since otherwise `pip install imas-simdb[server]` will produce a server that crashes on import.

Done!

* If the broker is down when `apply_async()` is called (the row is already committed as QUEUED), or a worker dies mid-COPYING (tasks are not `acks_late`, so the message is gone), the simulation will get stuck in a non-terminal state forever, and `delete_simulation` will return 409. The only recovery would seem to be manual DB surgery so we should probably add either an admin `force-delete` capability or a staleness timeout.

Added the force-delete feature you suggested. On top of that: more graceful error handling (a failed apply_async() now marks the simulation COPY_FAILED and returns 503 instead of leaving it stuck), a cleanup task that marks copy/validation jobs stuck in a non-terminal state as failed, and a timeout on the tasks themselves. The task timeout defaults to 1h, for very datasets we might want to want to raise this

* The watcher/notification behaviour is changed. Now `body.add_watcher` is ignored and the `send_email_task` never used - is this deliberate?

Fixed!

I think it looks reasonably complete now, I propose we merge it now and continue the test with actual use-cases with #91, #97 and #94 when updated accordingly.