Finix webhooks need three credentials and a public HTTPS URL; answer the {} probe with 200

Setting up Finix webhooks has two failure modes everyone hits. First, three credentials, not one: register a webhook username and password in the dashboard (Finix sends them on each delivery so you can verify the caller), and separately store the webhook signing key for HMAC verification. Second, the endpoint must be publicly reachable over HTTPS before you register it: Finix cannot deliver to YOUR_HOST or a private network, and a local dev URL fails silently. One more: when you first register, Finix sends an empty/{} verification delivery. Your handler must answer 200 to that probe and not try to parse an event from it.