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/User.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\User;
+
+use App\Shared\ValueObject\EmailAddress;
+
+class User
+{
+    public function __construct(
+        private int $id,
+        private EmailAddress $email,
+        private string $passwordHash,
+    ) {}
+
+    public function getId(): int
+    {
+        return $this->id;
+    }
+
+    public function getEmail(): EmailAddress
+    {
+        return $this->email;
+    }
+
+    public function getPasswordHash(): string
+    {
+        return $this->passwordHash;
+    }
+}