Set CRM order status to PACKED after Checkbox accepts the receipt

- ExoCrmClient.set_status: live SetStatus needs {Orders: [id], Status} and
  replies per order, unlike the documented {ID, Status}
- receipts.crm_status_set_at (migration 0006); set right after creation,
  retried by cron, row-locked to avoid a repeat PACKED overwriting a newer status
- CRM errors under capitalized 'Errors' and non-JSON replies are reported

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/crm/stub_client.py

@@ -40,6 +40,11 @@ _FIXTURE_ORDERS: list[dict] = [
 class StubCrmClient:
     def __init__(self, orders: list[dict] | None = None) -> None:
         self._orders = orders if orders is not None else _FIXTURE_ORDERS
+        # order_id → статус, выставленный через set_status (для проверок в тестах).
+        self.statuses: dict[str, str] = {}
+
+    async def set_status(self, *, order_id: str, status: str) -> None:
+        self.statuses[order_id] = status
 
     async def get_orders(self, *, status: str) -> list[OrderOut]:
         return [