implement CreatePost use case

trims title and body, rejects empty (post-trim) values with
BadRequest. supplies createdAt from injected Clock. persists
through PostRepository->create and returns the resulting Post.
44 tests pass.

backend/app/Post/UseCases/CreatePost/CreatePostRequest.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Post\UseCases\CreatePost;
+
+class CreatePostRequest
+{
+    public function __construct(
+        public int $userId,
+        public ?string $title,
+        public ?string $body,
+    ) {}
+}