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/CreateUserDto.php

@@ -3,12 +3,15 @@
 namespace App\User;
 
 use App\Shared\ValueObject\EmailAddress;
+use DateTimeImmutable;
 
 readonly class CreateUserDto
 {
     public function __construct(
         public EmailAddress $email,
+        public string $displayName,
         public string $passwordHash,
         public bool $isAdmin,
+        public ?DateTimeImmutable $emailConfirmedAt,
     ) {}
 }