Update Purchase Order Line Items

Operates on Purchase Orders. In the Zuper client app this module is labeled "Purchase Order" for most companies, and "Material Order" for companies in the roofing industry — the API and data shape are identical either way. Purchase Orders and Work Orders (Service Orders) share the exact same underlying schema and endpoints; they're distinguished only by the purchase_order_type field (PURCHASE_ORDER here) and the /purchase_orders vs /service_orders route prefix. Records fulfillment/receiving progress against a Purchase Order's line items — recomputes fulfilled_quantity, outstanding_quantity, per-item status, and rolls up the order's total_price if unit_price changed. This endpoint exists only for Purchase Orders / Material Orders — there is no equivalent /service_orders/:uid/line_items route for Work Orders.