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);

backend/src/main/resources/db/migration/V9__backfill_user_accounts.sql

@@ -41,11 +41,13 @@ WHERE e.user_id IS NULL
 
 UPDATE customer c
 JOIN users u ON u.email = c.email
+   AND u.role = 'CUSTOMER'
 SET c.user_id = u.id
 WHERE c.user_id IS NULL;
 
 UPDATE employee e
 JOIN users u ON u.email = e.email
+   AND u.role IN ('STAFF', 'ADMIN')
 SET e.user_id = u.id
 WHERE e.user_id IS NULL;