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/public/index.php

@@ -0,0 +1,19 @@
+<?php
+
+use DI\Bridge\Slim\Bridge;
+use Slim\App;
+
+(static function (): void {
+    $root = dirname(__DIR__);
+
+    require $root.'/vendor/autoload.php';
+
+    $container = require $root.'/config/container.php';
+
+    /** @var App $app */
+    $app = Bridge::create($container);
+
+    (require $root.'/config/routes.php')($app);
+
+    $app->run();
+})();