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/UserRepository.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\User;
+
+use App\Shared\ValueObject\EmailAddress;
+
+interface UserRepository
+{
+    public function create(CreateUserDto $dto): User;
+
+    public function findByEmail(EmailAddress $email): ?User;
+
+    public function find(int $id): ?User;
+}