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/Auth/SessionRepository.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Auth;
+
+interface SessionRepository
+{
+    public function create(CreateSessionDto $dto): Session;
+
+    public function findByToken(string $token): ?Session;
+
+    public function deleteByToken(string $token): void;
+}