Add background worker to process scheduled member deletions

Summary

In order to handle the deletion of many member records as efficiently as possible and without timeout, we are introducing a scheduled deletion process that will schedule member deletions via a new table and asynchronously process those scheduled deletions in a controlled and scalable manner.

See #467281 (comment 2135360209) for more details and previous discussion.

Proposal

To achieve this, the effort has been broken down into multiple steps:

#	Step	Issue/MR	Notes
1	Add a new model for tracking member deletion	#501247 (closed)
2	Add a background worker to process the scheduled deletions	this one!	👈🏼 we are here
3	Update relevant APIs	#501249 (closed)
4	Update the UI/UX	#480302 (closed)
5	Review and iterate	#501250 (closed)

Implementation detail

Create a background worker that processes records in the newly added table and deletes user memberships to a group hierarchy accordingly:

• One job (limited capacity worker?) running at a time
• Log relevant details (noted in this comment)
• Delete (or mark as complete?) the DB record
• We do something similar for AggregationSchedules via the PruneAggregationsScheduleWorker