Investigate Pull Mirroring improvements

Pull Mirroring is built over Import / Export and it may be time to reconsider the current architecture.

This issue is a investigation to determine what improvements could be made to Pull Mirroring to improve:

• performance
• resilience
• observability

It would also be useful to understand if Pull Mirroring is contributing to the Source Code error budget spend.

The scope of this issue is to determine what could be done to help achieve these goals, including adding observability, tuning, code splitting or a complete rewrite. If possible, also to define and weight additional issues.