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/CreatePost.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace App\Post\UseCases\CreatePost;
+
+use App\Auth\Clock;
+use App\Exceptions\BadRequestException;
+use App\Post\CreatePostDto;
+use App\Post\Post;
+use App\Post\PostRepository;
+
+class CreatePost
+{
+    public function __construct(
+        private PostRepository $postRepo,
+        private Clock $clock,
+    ) {}
+
+    /**
+     * @throws BadRequestException
+     */
+    public function execute(CreatePostRequest $request): Post
+    {
+        $title = $request->title === null ? '' : trim($request->title);
+        $body = $request->body === null ? '' : trim($request->body);
+
+        if ($title === '') {
+            throw new BadRequestException('title is required');
+        }
+        if ($body === '') {
+            throw new BadRequestException('body is required');
+        }
+
+        return $this->postRepo->create(new CreatePostDto(
+            userId: $request->userId,
+            title: $title,
+            body: $body,
+            createdAt: $this->clock->now(),
+        ));
+    }
+}

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,
+    ) {}
+}