Webhook handler for chargeback notifications.

Configure your endpoint at POST https://<your-server>/chargebacks to receive notifications when a chargeback is modified. Headers include:

• X-Api-Key: identifies which api-secret to use for signature verification
• X-Signature: HMAC-SHA256 digital signature
• X-Timestamp: unix-epoch timestamp
• X-Endpoint: the endpoint to which the request is made

You must respond with a 2xx HTTP status code, otherwise the notification will be resent.