Fix user linking

backend/src/main/java/com/petshop/backend/service/CustomerService.java

@@ -13,6 +13,9 @@ import org.springframework.data.domain.Pageable;
 import org.springframework.security.crypto.password.PasswordEncoder;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
+import org.springframework.web.server.ResponseStatusException;
+
+import static org.springframework.http.HttpStatus.CONFLICT;
 
 @Service
 public class CustomerService {
@@ -57,6 +60,12 @@ public class CustomerService {
         customer = customerRepository.save(customer);
         Customer savedCustomer = customer;
         User user = userRepository.findByEmail(savedCustomer.getEmail())
+                .map(existing -> {
+                    if (existing.getRole() != User.Role.CUSTOMER) {
+                        throw new ResponseStatusException(CONFLICT, "Email already exists for a different account type");
+                    }
+                    return existing;
+                })
                 .orElseGet(() -> createLinkedUser(savedCustomer));
 
         Customer linkedCustomer = userBusinessLinkageService.ensureLinkedCustomer(user);