Security: Introduce rate limit per endpoint

Describe the current behavior
Currently, Kaapi API endpoints have no rate limiting implemented. Any user or organization can make unlimited requests to any endpoint without restrictions. This creates several issues:

• A single user or organization can overwhelm the API with requests, consuming server resources and degrading performance for other users
• There's no protection against accidental or malicious misuse, such as runaway scripts or bot activity
• Celery operations like document uploads with provider integration or collection creation can cause celery job queue bloating

Describe the enhancement you'd like
Implement api level rate limiting across relevant API routes to ensure fair resource allocation and prevent misuse. The rate limiting should be conditional based on if it is a celery job endpoint or not. celery job endpoints such as collection creation, llm calls, etc should have lower number of api calls allowed per minute as compared to non celery job endpoints such as credential, onboarding, etc which can have higher number of api calls allowed per minute.

まず API ルートの定義と Celery を利用するエンドポイントを特定し、次に各レート制限を適用すべき操作を洗い出します。実装前に、1 分あたりのしきい値とレート制限の仕組みを明確にします。Celery エンドポイントと非 Celery エンドポイントでリクエストが異なる方法で制限され、キューまたはリソースの過負荷が防止されれば完了です。