implement ListUserPosts use case

validates userId > 0, delegates to PostRepository->findByUserId.
54 tests pass.

backend/app/Post/UseCases/ListUserPosts/ListUserPosts.php

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

backend/app/Post/UseCases/ListUserPosts/ListUserPostsRequest.php

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