Fixed Google Calendar sync dropping recurring interview slots

What we fixed

A timezone-handling bug in our recurring-event sync (visible on cohorts based in São Paulo, but theoretically broader) was producing instance arrays one slot short on every 4th occurrence. The missing slots existed in our database but never made it to Google Calendar.

What changed

• The recurrence-expansion code path now uses the IANA timezone of the program's primary location consistently from end to end.
• A back-fill job has re-emitted the missing slots to every affected calendar. Reviewers should see them on the next sync without taking any action.
• New monitoring will catch any future drift between expected and emitted slot counts within an hour.