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/client.py

@@ -0,0 +1,18 @@
+"""Protocol клиента Nova Poshta — позволяет подменять реализацию в тестах.
+
+См. `StubNovaPoshtaClient`.
+"""
+
+from __future__ import annotations
+
+from typing import Protocol
+
+from app.schemas.tracking import TrackingStatusOut
+
+
+class NovaPoshtaError(Exception):
+    """NP API ответил `success: false` (см. поле `errors` в ответе)."""
+
+
+class NovaPoshtaClient(Protocol):
+    async def get_statuses(self, *, waybill_numbers: list[str]) -> list[TrackingStatusOut]: ...