Add Checkbox ETTN receipts for Nova Poshta COD waybills

Cashier creates an ETTN receipt in Checkbox bound to the TTN with payment
control; Checkbox fiscalizes it itself when the parcel is paid for.

- cash_registers (Fernet-encrypted license key / PIN) and receipts tables
- Checkbox HTTP client + stub (ETTN does not work on test registers)
- two-phase create via ARQ job, timeout reconciliation, cron status polling
- /receipts and /cash-registers API, audit records
- dashboard: per-order and bulk create, prepayment, cancel; cash registers page

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

backend/tests/test_orders_router.py

@@ -79,6 +79,13 @@ def _patch_orders_service(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setattr(orders_router.orders_service, "list_orders", fake_list)
     monkeypatch.setattr(orders_router.orders_service, "delete_order", fake_delete)
 
+    async def fake_latest(session: object, order_ids: list[str]) -> dict:
+        return {}
+
+    monkeypatch.setattr(
+        orders_router.receipts_service, "latest_receipts_by_order", fake_latest
+    )
+
     async def fake_record(session: object, **kwargs: object) -> None:
         return None