implement DeletePost use case

backend/app/Post/UseCases/DeletePost/DeletePost.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace App\Post\UseCases\DeletePost;
+
+use App\Exceptions\BadRequestException;
+use App\Exceptions\ForbiddenException;
+use App\Post\PostRepository;
+
+class DeletePost
+{
+    public function __construct(
+        private PostRepository $postRepo,
+    ) {}
+
+    /**
+     * @throws BadRequestException
+     * @throws ForbiddenException
+     */
+    public function execute(DeletePostRequest $request): void
+    {
+        if ($request->postId <= 0) {
+            throw new BadRequestException('postId must be positive');
+        }
+        if ($request->requesterId <= 0) {
+            throw new BadRequestException('requesterId must be positive');
+        }
+
+        $post = $this->postRepo->find($request->postId);
+        if ($post === null) {
+            return;
+        }
+
+        $isAuthor = $post->getUserId() === $request->requesterId;
+        if (! $isAuthor && ! $request->requesterIsAdmin) {
+            throw new ForbiddenException(
+                'requester is not allowed to delete this post'
+            );
+        }
+
+        $this->postRepo->delete($request->postId);
+    }
+}

backend/app/Post/UseCases/DeletePost/DeletePostRequest.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Post\UseCases\DeletePost;
+
+class DeletePostRequest
+{
+    public function __construct(
+        public int $postId,
+        public int $requesterId,
+        public bool $requesterIsAdmin,
+    ) {}
+}