Deliver a survey's responses to an https endpoint. Every delivery carries an HMAC X-Zigpoll-Signature header formatted t=<unix ms>,v1=<hex>, where v1 is HMAC-SHA256 over <t>.<raw request body>. Delivery is at-least-once and retried on failure, so dedupe on X-Zigpoll-Delivery-Id.

Returns the account's signing secret ONCE, in this response. No other call returns it — store it when you create the webhook.

Endpoints must be public https URLs; localhost, internal hosts and IP literals are rejected. Maximum 20 per survey. Requires admin access.