Scheduling: reschedule UID drift — mirror keeps original Cal.com UID

After a reschedule, the appointments mirror keeps the ORIGINAL Cal.com booking UID; Cal.com issues a new UID, so a later cancel (under the new UID) may not resolve against our mirror. Flagged in the 2026-07-04 vendor memo; tracked in launch-blockers P1. Fix: update the mirrored UID on reschedule webhook/sync + regression test covering reschedule→cancel.