Fix: Missing property casting (#989)

Author: daveearley

backend/app/Models/OrderPaymentPlatformFee.php

@@ -17,6 +17,7 @@ protected function getCastMap(): array
             'application_fee_gross_amount' => 'float',
             'application_fee_net_amount' => 'float',
             'application_fee_vat_amount' => 'float',
+            'application_fee_vat_rate' => 'float',
         ];
     }