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/services/task_queue.py

@@ -0,0 +1,53 @@
+"""Постановка задач ARQ worker'у из API.
+
+Пул Redis создаётся лениво при первой задаче, а не в lifespan: недоступный
+Redis не должен ронять старт API. Если задачу поставить не удалось, чек
+остаётся `pending`, и cron `poll_receipts` подхватит его в течение минуты.
+"""
+
+from __future__ import annotations
+
+from typing import Any, Protocol
+
+from arq import ArqRedis, create_pool
+from arq.connections import RedisSettings
+
+from app.core.config import settings
+from app.core.logging import get_logger
+
+log = get_logger(__name__)
+
+
+class TaskQueue(Protocol):
+    async def enqueue(self, function: str, *args: Any) -> None: ...
+
+
+class ArqTaskQueue:
+    def __init__(self) -> None:
+        self._pool: ArqRedis | None = None
+
+    async def enqueue(self, function: str, *args: Any) -> None:
+        try:
+            if self._pool is None:
+                self._pool = await create_pool(
+                    RedisSettings.from_dsn(settings.redis_url), retry=0
+                )
+            await self._pool.enqueue_job(function, *args)
+        except Exception as exc:  # noqa: BLE001 — см. докстринг модуля
+            log.warning("enqueue_failed", function=function, error=repr(exc))
+
+    async def close(self) -> None:
+        if self._pool is not None:
+            await self._pool.aclose()
+            self._pool = None
+
+
+_queue = ArqTaskQueue()
+
+
+def get_task_queue() -> TaskQueue:
+    return _queue
+
+
+async def close_task_queue() -> None:
+    await _queue.close()