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>

frontend/src/api/receipts.ts

@@ -0,0 +1,10 @@
+import { apiFetch } from '@/api/client'
+import type { Receipt, ReceiptCreateResponse, ReceiptRequestItem } from '@/features/receipts/types'
+
+export function createReceipts(items: ReceiptRequestItem[]): Promise<ReceiptCreateResponse> {
+  return apiFetch<ReceiptCreateResponse>('/receipts', { method: 'POST', body: { items } })
+}
+
+export function cancelReceipt(receiptId: string): Promise<Receipt> {
+  return apiFetch<Receipt>(`/receipts/${receiptId}/cancel`, { method: 'POST' })
+}