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/tests/Unit/Auth/UseCases/CreateSessionTest.php

@@ -46,8 +46,10 @@ class CreateSessionTest extends TestCase
         return new User(
             id: 7,
             email: new EmailAddress('user@example.com'),
+            displayName: 'user',
             passwordHash: 'hashed:irrelevant',
             isAdmin: false,
+            emailConfirmedAt: null,
         );
     }