implement auth middleware

Green phase: PSR-15 AuthMiddleware checks auth_token cookie, looks up session, validates expiry, sets user attribute on request or returns 401.

backend/app/Middleware/AuthMiddleware.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace App\Middleware;
+
+use App\Auth\Clock;
+use App\Auth\SessionRepository;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Server\MiddlewareInterface;
+use Psr\Http\Server\RequestHandlerInterface;
+use Slim\Psr7\Response;
+
+class AuthMiddleware implements MiddlewareInterface
+{
+    public const COOKIE_NAME = 'auth_token';
+
+    public function __construct(
+        private SessionRepository $sessionRepo,
+        private Clock $clock,
+    ) {}
+
+    public function process(
+        ServerRequestInterface $request,
+        RequestHandlerInterface $handler,
+    ): ResponseInterface {
+        $cookies = $request->getCookieParams();
+        $token = $cookies[self::COOKIE_NAME] ?? null;
+
+        if (! is_string($token) || $token === '') {
+            return $this->unauthorized();
+        }
+
+        $session = $this->sessionRepo->findByToken($token);
+
+        if ($session === null) {
+            return $this->unauthorized();
+        }
+
+        if ($session->isExpired($this->clock->now())) {
+            $this->sessionRepo->deleteByToken($token);
+
+            return $this->unauthorized();
+        }
+
+        $request = $request->withAttribute('user', $session->getUser());
+
+        return $handler->handle($request);
+    }
+
+    private function unauthorized(): ResponseInterface
+    {
+        $response = new Response(401);
+        $response->getBody()->write(
+            json_encode(['error' => 'unauthenticated']),
+        );
+
+        return $response->withHeader('Content-Type', 'application/json');
+    }
+}