extend User entity with displayname and email confirmation

Add display_name (unique) and email_confirmed_at columns plus
matching getters, DTO fields, repo methods (findByDisplayName,
update), and migration. Existing auth tests updated to construct
User with the new params.

backend/app/User/UserRepository.php

@@ -11,4 +11,8 @@ interface UserRepository
     public function find(int $id): ?User;
 
     public function findByEmail(EmailAddress $email): ?User;
+
+    public function findByDisplayName(string $displayName): ?User;
+
+    public function update(User $user): User;
 }