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/core/config.py

@@ -62,6 +62,9 @@ class Settings(BaseSettings):
     crm_shop_key: str = ""
     crm_sid: int = 1
 
+    # --- Nova Poshta ---
+    nova_poshta_api_key: str = ""
+
     @property
     def cors_origins(self) -> list[str]:
         """Список разрешённых origin'ов из строки через запятую."""