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/db/models/order.py

@@ -7,10 +7,11 @@
 
 from __future__ import annotations
 
+import uuid
 from datetime import datetime
 from typing import Any
 
-from sqlalchemy import BigInteger, Boolean, DateTime, String, Text
+from sqlalchemy import BigInteger, Boolean, DateTime, ForeignKey, String, Text
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -54,3 +55,9 @@ class Order(TimestampMixin, Base):
     np_status_code: Mapped[str | None] = mapped_column(String(16))
     np_cod_amount_kopecks: Mapped[int | None] = mapped_column(BigInteger)
     np_payment_status: Mapped[str | None] = mapped_column(String(32))
+
+    # Касса, чей ключ НП видит ТТН как свою (определяется при опросе статусов,
+    # см. `services/orders.sync_np_statuses`). От неё создаётся ЕТТН-чек.
+    cash_register_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("cash_registers.id", ondelete="SET NULL"), index=True
+    )