bench: notification log

We would like a place on cloudblue where there is a log of notifications and users can opt out of email notifications.

This would require a persistent object for notification.
I briefly looked at how notifications would be implemented for AusoceanTV, but they would be different for CloudBlue.
WIP design note for AusOceanTV (for inspiration): https://docs.google.com/document/d/1-mtW3qs0QGrZRvjYy5JEhNrqh23P8ag0J3vRsazc92M/edit?tab=t.0

The issue names no files, tests, or entry points. Start with the linked AusOceanTV WIP design note, then map how CloudBlue would persist notifications and handle email opt-outs. Done means a notification log exists and users can opt out of email notifications.

Written by the indexing model from the issue text.