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/client.py

@@ -15,4 +15,8 @@ class NovaPoshtaError(Exception):
 
 
 class NovaPoshtaClient(Protocol):
-    async def get_statuses(self, *, waybill_numbers: list[str]) -> list[TrackingStatusOut]: ...
+    """Ключ API передаётся в каждый вызов: у каждой кассы свой кабинет отправителя."""
+
+    async def get_statuses(
+        self, *, api_key: str, waybill_numbers: list[str]
+    ) -> list[TrackingStatusOut]: ...