implement ListRecentPosts use case

validates limit > 0 (zero or negative -> BadRequest), then
delegates to PostRepository->findRecent. 49 tests pass.

backend/app/Post/UseCases/ListRecentPosts/ListRecentPostsRequest.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace App\Post\UseCases\ListRecentPosts;
+
+class ListRecentPostsRequest
+{
+    public function __construct(
+        public int $limit,
+    ) {}
+}