Check in at a shift (assigned, shift window from 60 min before start to the end, exact place, inside its fence)
Body: client_attempt_id (8-80 chars, retry-safe) and location {latitude, longitude, accuracy_meters, captured_at}. 403 not_assigned; 409 outside_shift_window; 422 outside_radius|uncertain_location|low_accuracy|address_imprecise|invalid_location.
POST
https://api.channelautomation.com/api/field-sales/rep/shifts/{id}/checkin