Attaches purchase orders to a contract job (ERP-12219).

Body: an array of { orderNumber, balance, hours, expiration, startDate, endDate, isPrimary }; only orderNumber is required. Each PO is matched by orderNumber among the job's client's POs, or created for that client with the balance, hours and expiration given (an existing PO's values are never changed). It is then linked to the job with startDate/endDate. The entry with isPrimary true becomes the job's primary PO, the one the invoice shows; at most one entry may set it. With none set, the job keeps its primary, or the first entry becomes primary if it has none. POs already on the job and not in the body stay linked: this never detaches. Sending the same body again changes nothing. Returns the job's POs, as GET.