Bind Nova Poshta API key to cash register (#3)

Each cash register stores its own encrypted NP API key. Status polling uses
register keys and binds an order to the register whose key sees the TTN as
its own (PhoneSender present); ETTN receipts are created from that register.
Migration 0008 moves the old NOVA_POSHTA_API_KEY into the default register.

Closes #3

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/nova_poshta/np_client.py

@@ -4,7 +4,6 @@ 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
 
@@ -15,10 +14,9 @@ _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]:
+    async def get_statuses(
+        self, *, api_key: str, waybill_numbers: list[str]
+    ) -> list[TrackingStatusOut]:
         if not waybill_numbers:
             return []
         if len(waybill_numbers) > _MAX_DOCUMENTS_PER_REQUEST:
@@ -28,7 +26,7 @@ class NpTrackingClient:
             )
 
         body = {
-            "apiKey": self._api_key,
+            "apiKey": api_key,
             "modelName": "TrackingDocument",
             "calledMethod": "getStatusDocuments",
             "methodProperties": {