Update Work Order Status

Operates on Service Orders, shown in the Zuper client app as "Work Order" (in every industry). Work Orders share the exact same underlying schema and endpoints as Purchase Orders / Material Orders; they're distinguished only by the purchase_order_type field (SERVICE_ORDER here) and the /service_orders route prefix. Status transitions are validated against a fixed transition matrix — an unreachable transition is rejected with 400. WORK_COMPLETED is not a valid status for a Purchase Order / Material Order; use FULFILLED instead.