add Comment entity, dto, repository interface

backend/app/Comment/Comment.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Comment;
+
+use DateTimeImmutable;
+
+class Comment
+{
+    public function __construct(
+        private int $id,
+        private int $postId,
+        private int $userId,
+        private string $body,
+        private DateTimeImmutable $createdAt,
+    ) {}
+
+    public function getId(): int
+    {
+        return $this->id;
+    }
+
+    public function getPostId(): int
+    {
+        return $this->postId;
+    }
+
+    public function getUserId(): int
+    {
+        return $this->userId;
+    }
+
+    public function getBody(): string
+    {
+        return $this->body;
+    }
+
+    public function getCreatedAt(): DateTimeImmutable
+    {
+        return $this->createdAt;
+    }
+}