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;
+    }
+}

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;
+}

backend/app/Comment/CreateCommentDto.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Comment;
+
+use DateTimeImmutable;
+
+readonly class CreateCommentDto
+{
+    public function __construct(
+        public int $postId,
+        public int $userId,
+        public string $body,
+        public DateTimeImmutable $createdAt,
+    ) {}
+}