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/app/db/models/__init__.py

@@ -6,14 +6,19 @@ Alembic автогенерирует миграции по `Base.metadata`, по
 
 from app.db.base import Base
 from app.db.models.audit import AuditAction, AuditLog
+from app.db.models.cash_register import CashRegister
 from app.db.models.order import Order
+from app.db.models.receipt import Receipt, ReceiptStatus
 from app.db.models.user import RefreshToken, User, UserRole
 
 __all__ = [
     "AuditAction",
     "AuditLog",
     "Base",
+    "CashRegister",
     "Order",
+    "Receipt",
+    "ReceiptStatus",
     "RefreshToken",
     "User",
     "UserRole",