add domain layer, config, and entry point

Domain: User, Session, EmailAddress, DTOs, repositories, services (PasswordHasher, TokenGenerator, Clock). Config: PHP-DI container definitions and Slim routes. Entry point: public/index.php with slim-bridge.

backend/app/User/CreateUserDto.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace App\User;
+
+use App\Shared\ValueObject\EmailAddress;
+
+class CreateUserDto
+{
+    public function __construct(
+        public EmailAddress $email,
+        public string $passwordHash,
+    ) {}
+}