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/main.py

@@ -16,6 +16,7 @@ from app.api.v1.router import api_router
 from app.core.config import settings
 from app.core.logging import configure_logging, get_logger
 from app.db.session import engine
+from app.services.task_queue import close_task_queue
 
 log = get_logger(__name__)
 
@@ -25,6 +26,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     configure_logging()
     log.info("app_starting", environment=settings.environment, version=app.version)
     yield
+    await close_task_queue()
     await engine.dispose()
     log.info("app_stopped")