Add new table for scheduling member deletion

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	this one!	👈🏼 we are here
2	Add a background worker to process the scheduled deletions	#501248 (closed)
3	Update relevant APIs	#501249 (closed)
4	Update the UI/UX	#480302 (closed)
5	Review and iterate	#501250 (closed)

Implementation detail

Create a new model/table that will support unique rows for root namespaces and users:

• user_id
• namespace_id
• unique index on [user_id, namespace_id]