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/Session.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Auth;
+
+use App\User\User;
+use DateTimeImmutable;
+
+class Session
+{
+    public function __construct(
+        private string $token,
+        private User $user,
+        private DateTimeImmutable $createdAt,
+        private DateTimeImmutable $expiresAt,
+    ) {}
+
+    public function getToken(): string
+    {
+        return $this->token;
+    }
+
+    public function getUser(): User
+    {
+        return $this->user;
+    }
+
+    public function getCreatedAt(): DateTimeImmutable
+    {
+        return $this->createdAt;
+    }
+
+    public function getExpiresAt(): DateTimeImmutable
+    {
+        return $this->expiresAt;
+    }
+
+    public function isExpired(DateTimeImmutable $now): bool
+    {
+        return $now >= $this->expiresAt;
+    }
+}