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/config/routes.php

@@ -0,0 +1,17 @@
+<?php
+
+use App\Controllers\AuthController;
+use App\Middleware\AuthMiddleware;
+use Slim\App;
+use Slim\Routing\RouteCollectorProxy;
+
+return function (App $app): void {
+
+    $app->get('/me', [AuthController::class, 'me'])
+        ->add(AuthMiddleware::class);
+
+    $app->post('/login', [AuthController::class, 'login']);
+
+    $app->post('/logout', [AuthController::class, 'logout'])
+        ->add(AuthMiddleware::class);
+};