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/services/nova_poshta/stub_client.py

@@ -0,0 +1,31 @@
+"""Фикстурный клиент Nova Poshta для тестов — не ходит в сеть."""
+
+from __future__ import annotations
+
+from app.schemas.tracking import TrackingStatusOut
+
+_FIXTURE_STATUSES: dict[str, dict] = {
+    "20450123456789": {
+        "Number": "20450123456789",
+        "Status": "Відправлення отримано",
+        "StatusCode": "9",
+        "PaymentMethod": "Cash",
+        "ScheduledDeliveryDate": "22-09-2026 18:00:00",
+        "ActualDeliveryDate": "22-09-2026 15:30:00",
+        "AmountToPay": "1200.00",
+        "AfterpaymentOnGoodsCost": 1200,
+        "PaymentStatus": "Paid",
+    }
+}
+
+
+class StubNovaPoshtaClient:
+    def __init__(self, statuses: dict[str, dict] | None = None) -> None:
+        self._statuses = statuses if statuses is not None else _FIXTURE_STATUSES
+
+    async def get_statuses(self, *, waybill_numbers: list[str]) -> list[TrackingStatusOut]:
+        return [
+            TrackingStatusOut.model_validate(self._statuses[number])
+            for number in waybill_numbers
+            if number in self._statuses
+        ]