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>

backend/app/schemas/orders.py

@@ -78,6 +78,10 @@ class OrderRowOut(BaseModel):
     total_amount: str
     goods: list[OrderGoodOut]
     has_receipt: bool
+    np_status: str | None
+    np_status_code: str | None
+    np_cod_amount: str | None
+    np_payment_status: str | None
 
     @classmethod
     def from_order(cls, order: Order) -> OrderRowOut:
@@ -92,4 +96,12 @@ class OrderRowOut(BaseModel):
             total_amount=f"{order.total_amount_kopecks / 100:.2f}",
             goods=[OrderGoodOut.model_validate(good) for good in order.goods],
             has_receipt=order.receipt_created_at is not None,
+            np_status=order.np_status,
+            np_status_code=order.np_status_code,
+            np_cod_amount=(
+                f"{order.np_cod_amount_kopecks / 100:.2f}"
+                if order.np_cod_amount_kopecks is not None
+                else None
+            ),
+            np_payment_status=order.np_payment_status,
         )