Build Admin Attik support for cross-instance shared calendars

Build support in Admin Attik for inspectors who work across multiple instances to share one availability source so a booking in one instance blocks availability in the others.

Transcript Context

The discussion described inspectors working across brands and instances, especially in Florida and for Monument, where a booking in one instance should block the same inspector in another instance. The desired behavior is a single master calendar for a person who exists in multiple instances, so teams can book that inspector across brands without creating overlapping availability.

A possible implementation path mentioned in the discussion was to leverage ATT-1357 so bookings can round-trip through Google Calendar sync and come back as blocked time in the other instance. The requirement is to block time only, not create a duplicate job in the second instance.

This should be built with Admin Attik as the control point for managing cross-instance identity and shared availability.