implement SetFeaturedPost use case

backend/app/Post/UseCases/SetFeaturedPost/SetFeaturedPost.php

@@ -0,0 +1,69 @@
+<?php
+
+namespace App\Post\UseCases\SetFeaturedPost;
+
+use App\Exceptions\BadRequestException;
+use App\Exceptions\ForbiddenException;
+use App\Post\Post;
+use App\Post\PostRepository;
+use DomainException;
+
+class SetFeaturedPost
+{
+    private const VALID_SLOTS = [1, 2];
+
+    public function __construct(
+        private PostRepository $postRepo,
+    ) {}
+
+    /**
+     * @throws BadRequestException
+     * @throws ForbiddenException
+     * @throws DomainException
+     */
+    public function execute(SetFeaturedPostRequest $request): Post
+    {
+        if (! $request->requesterIsAdmin) {
+            throw new ForbiddenException(
+                'only admins can feature a post'
+            );
+        }
+        if ($request->postId <= 0) {
+            throw new BadRequestException('postId must be positive');
+        }
+        if (! in_array($request->slot, self::VALID_SLOTS, true)) {
+            throw new BadRequestException(
+                'slot must be 1 or 2'
+            );
+        }
+
+        $post = $this->postRepo->find($request->postId);
+        if ($post === null) {
+            throw new DomainException('post not found');
+        }
+
+        $existingInSlot = $this->postRepo->findByFeatureSlot($request->slot);
+        if (
+            $existingInSlot !== null
+            && $existingInSlot->getId() !== $post->getId()
+        ) {
+            $this->postRepo->update(new Post(
+                id: $existingInSlot->getId(),
+                userId: $existingInSlot->getUserId(),
+                title: $existingInSlot->getTitle(),
+                body: $existingInSlot->getBody(),
+                createdAt: $existingInSlot->getCreatedAt(),
+                featureSlot: null,
+            ));
+        }
+
+        return $this->postRepo->update(new Post(
+            id: $post->getId(),
+            userId: $post->getUserId(),
+            title: $post->getTitle(),
+            body: $post->getBody(),
+            createdAt: $post->getCreatedAt(),
+            featureSlot: $request->slot,
+        ));
+    }
+}

backend/app/Post/UseCases/SetFeaturedPost/SetFeaturedPostRequest.php

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