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/checkbox/client.py

@@ -0,0 +1,61 @@
+"""Protocol клиента Checkbox — позволяет подменять реализацию в тестах и локально.
+
+См. `StubCheckboxClient`. Выбор реализации — `get_checkbox_client()` ниже,
+единственное место, читающее `CHECKBOX_USE_STUB`.
+"""
+
+from __future__ import annotations
+
+import uuid
+from dataclasses import dataclass
+from functools import lru_cache
+from typing import Any, Protocol
+
+from app.core.config import settings
+from app.schemas.checkbox import EttnOut
+
+
+class CheckboxError(Exception):
+    """Checkbox отклонил запрос (4xx) — повтор с тем же телом не поможет."""
+
+
+class CheckboxUnavailableError(CheckboxError):
+    """Сеть, таймаут или 5xx — исход запроса неизвестен, можно повторить."""
+
+
+@dataclass(frozen=True)
+class CheckboxCredentials:
+    """Расшифрованные доступы одной кассы. Живут только в памяти."""
+
+    cash_register_id: uuid.UUID
+    license_key: str
+    pin_code: str
+
+
+class CheckboxClient(Protocol):
+    async def sign_in(self, creds: CheckboxCredentials) -> None: ...
+
+    async def create_ettn(self, creds: CheckboxCredentials, body: dict[str, Any]) -> EttnOut: ...
+
+    async def get_ettn(self, creds: CheckboxCredentials, ettn_id: str) -> EttnOut: ...
+
+    async def find_ettn(
+        self, creds: CheckboxCredentials, waybill_number: str
+    ) -> EttnOut | None: ...
+
+    async def delete_ettn(self, creds: CheckboxCredentials, ettn_id: str) -> None: ...
+
+
+@lru_cache
+def get_checkbox_client() -> CheckboxClient:
+    """Один экземпляр на процесс: внутри — кэш токенов кассиров."""
+    if settings.checkbox_use_stub:
+        if settings.is_production:
+            raise RuntimeError("CHECKBOX_USE_STUB=true запрещён в production")
+        from app.services.checkbox.stub_client import StubCheckboxClient
+
+        return StubCheckboxClient(auto_complete_after=2)
+
+    from app.services.checkbox.http_client import HttpCheckboxClient
+
+    return HttpCheckboxClient(settings)