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/np_client.py

@@ -0,0 +1,49 @@
+"""Реальный клиент Nova Poshta (`TrackingDocument.getStatusDocuments`)."""
+
+from __future__ import annotations
+
+import httpx
+
+from app.core.config import Settings
+from app.schemas.tracking import TrackingStatusOut
+from app.services.nova_poshta.client import NovaPoshtaError
+
+_API_URL = "https://api.novaposhta.ua/v2.0/json/"
+
+# NP отклоняет запросы с более чем 100 накладными за раз.
+_MAX_DOCUMENTS_PER_REQUEST = 100
+
+
+class NpTrackingClient:
+    def __init__(self, settings: Settings) -> None:
+        self._api_key = settings.nova_poshta_api_key
+
+    async def get_statuses(self, *, waybill_numbers: list[str]) -> list[TrackingStatusOut]:
+        if not waybill_numbers:
+            return []
+        if len(waybill_numbers) > _MAX_DOCUMENTS_PER_REQUEST:
+            raise NovaPoshtaError(
+                f"Слишком много ТТН за один запрос: {len(waybill_numbers)} "
+                f"(максимум {_MAX_DOCUMENTS_PER_REQUEST})"
+            )
+
+        body = {
+            "apiKey": self._api_key,
+            "modelName": "TrackingDocument",
+            "calledMethod": "getStatusDocuments",
+            "methodProperties": {
+                "Documents": [{"DocumentNumber": number, "Phone": ""} for number in waybill_numbers]
+            },
+        }
+
+        async with httpx.AsyncClient(timeout=30) as client:
+            response = await client.post(_API_URL, json=body)
+            response.raise_for_status()
+            data = response.json()
+
+        if not data.get("success"):
+            errors = data.get("errors") or []
+            message = "; ".join(str(error) for error in errors)
+            raise NovaPoshtaError(f"NP вернул ошибку: {message or 'неизвестная ошибка'}")
+
+        return [TrackingStatusOut.model_validate(item) for item in data.get("data", [])]