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/database/migrations/2026_05_06_000000_create_users_table.php

@@ -11,8 +11,10 @@ return new class extends Migration
         Schema::create('users', function (Blueprint $table) {
             $table->id();
             $table->string('email')->unique();
+            $table->string('display_name')->unique();
             $table->string('password_hash');
             $table->boolean('is_admin')->default(false);
+            $table->dateTime('email_confirmed_at')->nullable();
         });
     }