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/db/models/cash_register.py

@@ -0,0 +1,39 @@
+"""Кассы (ПРРО) Checkbox.
+
+Ключ лицензии и PIN кассира хранятся только в зашифрованном виде
+(`app/core/crypto.py`): утечка дампа БД не должна давать доступ к кассе.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from sqlalchemy import Boolean, Index, String, text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class CashRegister(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    __tablename__ = "cash_registers"
+
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    fiscal_number: Mapped[str | None] = mapped_column(String(64))
+    license_key_enc: Mapped[str] = mapped_column(String(512), nullable=False)
+    cashier_pin_enc: Mapped[str] = mapped_column(String(512), nullable=False)
+    # Коды налоговых ставок Checkbox для всех товаров чека; пусто — поле `tax`
+    # не передаётся (неплательщик ПДВ).
+    tax_codes: Mapped[list[Any]] = mapped_column(JSONB, nullable=False, default=list)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    is_default: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+
+    __table_args__ = (
+        # Касса по умолчанию может быть только одна.
+        Index(
+            "uq_cash_registers_default",
+            "is_default",
+            unique=True,
+            postgresql_where=text("is_default"),
+        ),
+    )