implement CreateComment use case

backend/app/Comment/UseCases/CreateComment/CreateComment.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace App\Comment\UseCases\CreateComment;
+
+use App\Auth\Clock;
+use App\Comment\Comment;
+use App\Comment\CommentRepository;
+use App\Comment\CreateCommentDto;
+use App\Exceptions\BadRequestException;
+use App\Post\PostRepository;
+use DomainException;
+
+class CreateComment
+{
+    public function __construct(
+        private CommentRepository $commentRepo,
+        private PostRepository $postRepo,
+        private Clock $clock,
+    ) {}
+
+    /**
+     * @throws BadRequestException
+     * @throws DomainException
+     */
+    public function execute(CreateCommentRequest $request): Comment
+    {
+        if ($request->postId <= 0) {
+            throw new BadRequestException('postId must be positive');
+        }
+        if ($request->userId <= 0) {
+            throw new BadRequestException('userId must be positive');
+        }
+        $body = $request->body === null ? '' : trim($request->body);
+        if ($body === '') {
+            throw new BadRequestException('body is required');
+        }
+
+        if ($this->postRepo->find($request->postId) === null) {
+            throw new DomainException('post not found');
+        }
+
+        return $this->commentRepo->create(new CreateCommentDto(
+            postId: $request->postId,
+            userId: $request->userId,
+            body: $body,
+            createdAt: $this->clock->now(),
+        ));
+    }
+}

backend/app/Comment/UseCases/CreateComment/CreateCommentRequest.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Comment\UseCases\CreateComment;
+
+class CreateCommentRequest
+{
+    public function __construct(
+        public int $postId,
+        public int $userId,
+        public ?string $body,
+    ) {}
+}