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/audit.py

@@ -32,6 +32,10 @@ class AuditAction(str):
     USER_UPDATED = "user.updated"
     USER_DEACTIVATED = "user.deactivated"
     ORDER_DELETED = "order.deleted"
+    CASH_REGISTER_CREATED = "cash_register.created"
+    CASH_REGISTER_UPDATED = "cash_register.updated"
+    RECEIPT_CREATE_REQUESTED = "receipt.create_requested"
+    RECEIPT_CANCELLED = "receipt.cancelled"
 
 
 class AuditLog(UUIDPrimaryKeyMixin, Base):