fix: remove t.Parallel from VAT tests to eliminate deadlock flakiness

backend/handlers/payments/vat_test.go

@@ -2994,14 +2994,21 @@ func TestVAT_DisableVATRegistration_Lifecycle(t *testing.T) {
 		t.Fatalf("phase 3: expected 200, got %d: %s", w3.Code, w3.Body.String())
 	}
 
-	// Verify NO VAT on the second payment (the latest cash payment)
+	var phase3Resp CheckoutResponse
+	if err := json.Unmarshal(w3.Body.Bytes(), &phase3Resp); err != nil {
+		t.Fatalf("phase 3: failed to parse response: %v", err)
+	}
+
+	// Verify NO VAT on the second payment (query by specific payment ID
+	// avoids non-deterministic ORDER BY created_at DESC when all payments
+	// share the same transaction start time from NOW()).
 	var vat3 sql.NullFloat64
 	var net3 sql.NullFloat64
 	var vatApplicable3 bool
 	var amount3 float64
-	err = tx.QueryRow(ctx, `SELECT amount, is_vat_applicable, vat_amount, net_amount FROM payments WHERE booking_id = $1 AND payment_method = 'cash' ORDER BY created_at DESC LIMIT 1`, bookingID).Scan(&amount3, &vatApplicable3, &vat3, &net3)
+	err = tx.QueryRow(ctx, `SELECT amount, is_vat_applicable, vat_amount, net_amount FROM payments WHERE id = $1`, phase3Resp.CheckoutID).Scan(&amount3, &vatApplicable3, &vat3, &net3)
 	if err != nil {
-		t.Fatalf("phase 3: failed to query latest payment: %v", err)
+		t.Fatalf("phase 3: failed to query payment: %v", err)
 	}
 
 	if vatApplicable3 {
@@ -3138,14 +3145,21 @@ func TestVAT_DisableAndReEnable_Lifecycle(t *testing.T) {
 		t.Fatalf("phase 2: expected 200, got %d: %s", w2.Code, w2.Body.String())
 	}
 
-	// Get the latest cash payment (the second one, without VAT)
+	var phase2Resp CheckoutResponse
+	if err := json.Unmarshal(w2.Body.Bytes(), &phase2Resp); err != nil {
+		t.Fatalf("phase 2: failed to parse response: %v", err)
+	}
+
+	// Get the specific payment by its ID from the response (avoiding
+	// non-deterministic ORDER BY created_at DESC when NOW() returns the same
+	// transaction start time for all inserts within a transaction).
 	var p2VAT sql.NullFloat64
 	var p2Net sql.NullFloat64
 	var p2VATApplicable bool
 	var p2Amount float64
-	err = tx.QueryRow(ctx, `SELECT amount, is_vat_applicable, vat_amount, net_amount FROM payments WHERE booking_id = $1 AND payment_method = 'cash' ORDER BY created_at DESC LIMIT 1`, bookingID).Scan(&p2Amount, &p2VATApplicable, &p2VAT, &p2Net)
+	err = tx.QueryRow(ctx, `SELECT amount, is_vat_applicable, vat_amount, net_amount FROM payments WHERE id = $1`, phase2Resp.CheckoutID).Scan(&p2Amount, &p2VATApplicable, &p2VAT, &p2Net)
 	if err != nil {
-		t.Fatalf("phase 2: failed to query latest payment: %v", err)
+		t.Fatalf("phase 2: failed to query payment: %v", err)
 	}
 
 	if p2VATApplicable {
@@ -3182,13 +3196,20 @@ func TestVAT_DisableAndReEnable_Lifecycle(t *testing.T) {
 		t.Fatalf("phase 3: expected 200, got %d: %s", w3.Code, w3.Body.String())
 	}
 
-	// Get the latest cash payment (the third one, with VAT re-enabled)
+	var phase3Resp CheckoutResponse
+	if err := json.Unmarshal(w3.Body.Bytes(), &phase3Resp); err != nil {
+		t.Fatalf("phase 3: failed to parse response: %v", err)
+	}
+
+	// Get the specific payment by its ID from the response (avoiding
+	// non-deterministic ORDER BY created_at DESC when NOW() returns the same
+	// transaction start time for all inserts within a transaction).
 	var p3VAT sql.NullFloat64
 	var p3Net sql.NullFloat64
 	var p3VATApplicable bool
-	err = tx.QueryRow(ctx, `SELECT is_vat_applicable, vat_amount, net_amount FROM payments WHERE booking_id = $1 AND payment_method = 'cash' ORDER BY created_at DESC LIMIT 1`, bookingID).Scan(&p3VATApplicable, &p3VAT, &p3Net)
+	err = tx.QueryRow(ctx, `SELECT is_vat_applicable, vat_amount, net_amount FROM payments WHERE id = $1`, phase3Resp.CheckoutID).Scan(&p3VATApplicable, &p3VAT, &p3Net)
 	if err != nil {
-		t.Fatalf("phase 3: failed to query latest payment: %v", err)
+		t.Fatalf("phase 3: failed to query payment: %v", err)
 	}
 
 	if !p3VATApplicable {