fix: add missing err checks and fix loop termination in tests

backend/handlers/payments/vat_test.go

@@ -3357,6 +3357,9 @@ func TestVAT_RoundingConsistency(t *testing.T) {
 			var netAmount sql.NullFloat64
 			var isVATApplicable bool
 			err = tx.QueryRow(ctx, `SELECT is_vat_applicable, vat_amount, net_amount FROM payments WHERE id = $1`, paymentID).Scan(&isVATApplicable, &vatAmount, &netAmount)
+			if err != nil {
+				t.Fatalf("failed to query payment VAT fields: %v", err)
+			}
 
 			if !isVATApplicable {
 				t.Fatal("expected VAT to be applicable")