Adds NpTrackingClient (Protocol + real/stub impls) and an ARQ worker that polls Nova Poshta every minute for orders without a receipt, writing status, net COD amount (Контроль оплати), and payment status onto the order. Surfaced in the orders table and detail modal. Marks plan stages 3-4 done in README. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>