arreglados varios temas además del DL (redsys, etc)

src/main/java/com/imprimelibros/erp/payments/PaymentService.java

@@ -273,6 +273,14 @@ public class PaymentService {
             throw new IllegalStateException("Error al solicitar la devolución a Redsys", e);
         }
 
+        // 🔧 NORMALIZAR ANTES DE GUARDAR
+        if (gatewayRefundId != null) {
+            gatewayRefundId = gatewayRefundId.trim();
+            if (gatewayRefundId.isEmpty() || "000000".equals(gatewayRefundId)) {
+                gatewayRefundId = null; // → múltiples NULL NO rompen el UNIQUE
+            }
+        }
+
         PaymentTransaction tx = new PaymentTransaction();
         tx.setPayment(p);
         tx.setType(PaymentTransactionType.REFUND);