implement logout use case

Green phase: Logout calls deleteByToken on SessionRepository. No-op for non-existent tokens.

backend/app/Auth/UseCases/Logout/Logout.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace App\Auth\UseCases\Logout;
+
+use App\Auth\SessionRepository;
+
+class Logout
+{
+    public function __construct(
+        private SessionRepository $sessionRepo,
+    ) {}
+
+    public function execute(string $token): void
+    {
+        $this->sessionRepo->deleteByToken($token);
+    }
+}