fix: remove t.Parallel from VAT lifecycle test

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/payments/vat_test.go

@@ -2935,7 +2935,6 @@ func TestApplyVATToBookingPayment_OnTheHouse_Skip(t *testing.T) {
 // subsequent payments from having VAT applied, and that PaymentSummary only
 // reflects VAT from payments made while VAT was enabled.
 func TestVAT_DisableVATRegistration_Lifecycle(t *testing.T) {
-	t.Parallel()
 	ctx, tx := testutils.SetupTestTx(t)
 
 	_, err := tx.Exec(ctx, `UPDATE business_settings SET is_vat_registered = TRUE, default_vat_rate = 20.00`)