Configure a Cloudinary upload to run eager transformations asynchronously and notify a backend webhook when each one finishes

Steps

1. Pass an `eager` parameter on the upload call listing the transformation(s) to pre-generate as derived assets
2. Set `eager_async: true` so the upload call returns immediately after storing the original, while the listed eager transformations process in the background
3. Set `eager_notification_url` to an HTTPS endpoint you control; Cloudinary POSTs a JSON payload there when each eager transformation completes, separate from any general `notification_url`
4. On the receiving endpoint, verify the request is genuinely from Cloudinary by validating the `X-Cld-Signature` and `X-Cld-Timestamp` headers against the payload and your API secret before trusting the derived-asset URLs
5. Persist the derived asset URLs from the notification payload rather than assuming eager transformations are ready immediately after the initial upload response