Add Nova Poshta tracking: TTN status, COD amount, payment status

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>

frontend/src/features/orders/OrderDetailModal.tsx

@@ -27,6 +27,24 @@ export function OrderDetailModal({ order, onClose }: OrderDetailModalProps) {
             <span className="order-modal-label">Номер ТТН</span>
             <span>{order.waybill_number || '—'}</span>
           </div>
+          <div>
+            <span className="order-modal-label">Статус ТТН</span>
+            <span>{order.np_status || 'Нет данных'}</span>
+          </div>
+          <div>
+            <span className="order-modal-label">Наложенный платёж</span>
+            <span>{order.np_cod_amount ? `${order.np_cod_amount} ₴` : '—'}</span>
+          </div>
+          <div>
+            <span className="order-modal-label">Оплачено</span>
+            <span>
+              {order.np_payment_status === 'Payed'
+                ? 'Да'
+                : order.np_payment_status === 'NeedPayment'
+                  ? 'Нет'
+                  : '—'}
+            </span>
+          </div>
           <div>
             <span className="order-modal-label">Клиент</span>
             <span>{order.recipient_name || '—'}</span>