add Comment entity, dto, repository interface

backend/app/Comment/CommentRepository.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace App\Comment;
+
+interface CommentRepository
+{
+    public function create(CreateCommentDto $dto): Comment;
+
+    public function find(int $id): ?Comment;
+
+    /**
+     * @return Comment[]
+     */
+    public function findByPostId(int $postId): array;
+
+    public function delete(int $id): void;
+}