implement Logout use case

thin pass-through to SessionRepository->deleteByToken. test
green; 33 tests pass.

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);
+    }
+}