Draft: PoC Seat Assignment Service

What does this MR do and why?

Add seat assignment service for seat management

This change adds an event, publisher, and subscriber to upsert seat assignments triggered by a callback on the Member model.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #567810 (closed)