implement ListRecentPosts use case

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

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

@@ -0,0 +1,28 @@
+<?php
+
+namespace App\Post\UseCases\ListRecentPosts;
+
+use App\Exceptions\BadRequestException;
+use App\Post\Post;
+use App\Post\PostRepository;
+
+class ListRecentPosts
+{
+    public function __construct(
+        private PostRepository $postRepo,
+    ) {}
+
+    /**
+     * @return Post[]
+     *
+     * @throws BadRequestException
+     */
+    public function execute(ListRecentPostsRequest $request): array
+    {
+        if ($request->limit <= 0) {
+            throw new BadRequestException('limit must be positive');
+        }
+
+        return $this->postRepo->findRecent($request->limit);
+    }
+}

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,
+    ) {}
+}