add creation

自由创作：新增 Creation 模型（继承 Task，display 自动递增）与
/api/creation/ 路由，支持增删改和代码保存，只有创作者本人和超管可访问。
自由创作不进入提交/评分体系；prompt 的两个 WebSocket consumer
连接时校验 can_access_task。补充 task/tests.py 覆盖上述行为。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

api/urls.py

@@ -26,6 +26,7 @@ api = NinjaAPI()
 api.add_router("account/", "account.api.router")
 api.add_router("tutorial/", "task.tutorial.router")
 api.add_router("challenge/", "task.challenge.router")
+api.add_router("creation/", "task.creation.router")
 api.add_router("submission/", "submission.api.router")
 api.add_router("upload/", "utils.upload.router")
 api.add_router("assets/", "task.assets.router")