Complete a visit guest's check-in

Marks the guest identified by guestId as checked in for their parent Visit. The action is idempotent — calling it again on a guest that is already checked in is a no-op and returns the same 204 No Content. No request body.

Use this when a guest physically arrives at the venue: it finalises the check-in started at the kiosk or front desk, marks any open intake-form sessions as complete for the guest, and propagates the check-in status to downstream integrations.

Requires the RESERVATIONS_MANAGE permission on the visit's site.