implement DeleteComment use case

backend/app/Comment/UseCases/DeleteComment/DeleteComment.php

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

backend/app/Comment/UseCases/DeleteComment/DeleteCommentRequest.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Comment\UseCases\DeleteComment;
+
+class DeleteCommentRequest
+{
+    public function __construct(
+        public int $commentId,
+        public int $requesterId,
+        public bool $requesterIsAdmin,
+    ) {}
+}