Make WorkerActivity.process_id mandatory

Refs https://gitlab.com/teklia/arkindex/backend/-/issues/1350

This will require first to look on prod & demo how many activities without process we have, and if they are important (i guess we can delete most of them anyway).

Making WorkerActivity.process_id non-nullable will make the assignation cleaner & more explicit in the code, avoiding issues where we have no idea where the activities went and get re-assigned later on.