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/schemas/checkbox.py

@@ -0,0 +1,32 @@
+"""Схемы Checkbox API для ЕТТН-чеков (`/api/v1/ettn`).
+
+Тело запроса собирается как обычный dict в `app/services/receipts.py` —
+его снимок сохраняется в `receipts.request_body` без преобразований.
+Здесь — только разбор ответа (`BaseEttnResponse`).
+"""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class EttnStatus:
+    """Статусы ЕТТН-чека в Checkbox (`OrderStatus`)."""
+
+    CREATED = "CREATED"
+    CANCELLED = "CANCELLED"
+    RECEIPT_ERROR = "RECEIPT_ERROR"
+    DONE = "DONE"
+    DONE_WITHOUT_SMS = "DONE_WITHOUT_SMS"
+    RETURNED = "RETURNED"
+
+
+class EttnOut(BaseModel):
+    model_config = ConfigDict(populate_by_name=True)
+
+    id: str
+    status: str
+    ettn_number: str = Field(alias="ettnNumber")
+    total_sum: int | None = Field(default=None, alias="totalSum")
+    receipt_id: str | None = Field(default=None, alias="receiptId")
+    raw_error: str | None = Field(default=None, alias="rawError")