implement GetPost use case

validates id > 0, delegates to PostRepository->find. 58 tests
pass.

backend/app/Post/UseCases/GetPost/GetPost.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Post\UseCases\GetPost;
+
+use App\Exceptions\BadRequestException;
+use App\Post\Post;
+use App\Post\PostRepository;
+
+class GetPost
+{
+    public function __construct(
+        private PostRepository $postRepo,
+    ) {}
+
+    /**
+     * @throws BadRequestException
+     */
+    public function execute(int $id): ?Post
+    {
+        if ($id <= 0) {
+            throw new BadRequestException('id must be positive');
+        }
+
+        return $this->postRepo->find($id);
+    }
+}