implement ListCommentsForPost use case

Renames seed() helper to seedComment() to avoid clashing with
Illuminate\Foundation\Testing\TestCase::seed().

backend/app/Comment/UseCases/ListCommentsForPost/ListCommentsForPost.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace App\Comment\UseCases\ListCommentsForPost;
+
+use App\Comment\Comment;
+use App\Comment\CommentRepository;
+use App\Exceptions\BadRequestException;
+
+class ListCommentsForPost
+{
+    public function __construct(
+        private CommentRepository $commentRepo,
+    ) {}
+
+    /**
+     * @return Comment[]
+     *
+     * @throws BadRequestException
+     */
+    public function execute(ListCommentsForPostRequest $request): array
+    {
+        if ($request->postId <= 0) {
+            throw new BadRequestException('postId must be positive');
+        }
+
+        return $this->commentRepo->findByPostId($request->postId);
+    }
+}

backend/app/Comment/UseCases/ListCommentsForPost/ListCommentsForPostRequest.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace App\Comment\UseCases\ListCommentsForPost;
+
+class ListCommentsForPostRequest
+{
+    public function __construct(
+        public int $postId,
+    ) {}
+}