Add router duplicate subnet validation

src/main/java/com/litoralregas/openvpn/router/RouterRepository.java

@@ -1,7 +1,10 @@
 package com.litoralregas.openvpn.router;
 
 import org.springframework.data.jpa.repository.JpaRepository;
+
 import java.util.UUID;
 
 public interface RouterRepository extends JpaRepository<Router, UUID> {
+
+    boolean existsByLanSubnet(String lanSubnet);
 }

src/main/java/com/litoralregas/openvpn/router/RouterService.java

@@ -20,6 +20,10 @@ public class RouterService {
     }
 
     public Router create(CreateRouterRequest request) {
+        if (repository.existsByLanSubnet(request.getLanSubnet())) {
+            throw new IllegalArgumentException("LAN subnet already exists: " + request.getLanSubnet());
+        }
+
         LocalDateTime now = LocalDateTime.now();
 
         Router router = new Router();