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/__init__.py

@@ -0,0 +1,4 @@
+"""Интеграция с Nova Poshta (статусы ТТН, сумма наложенного платежа).
+
+См. `client.py` за Protocol и `np_client.py`/`stub_client.py` за реализациями.
+"""