Adds a line (one maintenance job with its costs) to a repair order. Lines are used by accounts with repair order lines enabled; for those accounts, the order's amount_of_parts and amount_of_labor become the sum of its lines after each line is saved.
The repair order in data.order, as loaded before the change, and the line in data.line. Get the order again to read its current amount_of_parts and amount_of_labor. HTTP 200 is also used for failures: always check success. When it is false, the body is an ErrorUnauthorized (status_code 401 or 403).