implement ClearFeaturedPost use case

backend/app/Post/UseCases/ClearFeaturedPost/ClearFeaturedPost.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace App\Post\UseCases\ClearFeaturedPost;
+
+use App\Exceptions\BadRequestException;
+use App\Exceptions\ForbiddenException;
+use App\Post\Post;
+use App\Post\PostRepository;
+
+class ClearFeaturedPost
+{
+    public function __construct(
+        private PostRepository $postRepo,
+    ) {}
+
+    /**
+     * @throws BadRequestException
+     * @throws ForbiddenException
+     */
+    public function execute(ClearFeaturedPostRequest $request): void
+    {
+        if (! $request->requesterIsAdmin) {
+            throw new ForbiddenException(
+                'only admins can unfeature a post'
+            );
+        }
+        if ($request->postId <= 0) {
+            throw new BadRequestException('postId must be positive');
+        }
+
+        $post = $this->postRepo->find($request->postId);
+        if ($post === null) {
+            return;
+        }
+        if (! $post->isFeatured()) {
+            return;
+        }
+
+        $this->postRepo->update(new Post(
+            id: $post->getId(),
+            userId: $post->getUserId(),
+            title: $post->getTitle(),
+            body: $post->getBody(),
+            createdAt: $post->getCreatedAt(),
+            featureSlot: null,
+        ));
+    }
+}

backend/app/Post/UseCases/ClearFeaturedPost/ClearFeaturedPostRequest.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace App\Post\UseCases\ClearFeaturedPost;
+
+class ClearFeaturedPostRequest
+{
+    public function __construct(
+        public int $postId,
+        public bool $requesterIsAdmin,
+    ) {}
+}