Replace the ability to DELETE a purchase order with the ability to CANCEL it. Rename the DeletePurchaseOrder operation to CancelPurchaseOrder everywhere it appears (the service interface and implementation, the WebAPI controller action and its HTTP route, and the WebApp controller, service and interface). When cancelling, instead of soft-deleting via IsActive = false, set the order's status to Cancelled (Enum_Status = (int)PurchaseOrderStatus.Cancelled). Update user-facing text from delete/deleting to cancel/cancelling. Do NOT touch the separate PurchaseOrderLine delete operation.
