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/db/models/audit.py

@@ -36,6 +36,7 @@ class AuditAction(str):
     CASH_REGISTER_UPDATED = "cash_register.updated"
     RECEIPT_CREATE_REQUESTED = "receipt.create_requested"
     RECEIPT_CANCELLED = "receipt.cancelled"
+    ORDER_CRM_STATUS_SET = "order.crm_status_set"
 
 
 class AuditLog(UUIDPrimaryKeyMixin, Base):