Function webhook_handler 

pub async fn webhook_handler(
    __arg0: State<Arc<WebhookState>>,
    __arg1: Path<String>,
    headers: HeaderMap,
    body: Bytes,
) -> impl IntoResponse

Handle webhook requests.

This handler:

1. Looks up webhook by path
2. Validates signature if configured
3. Checks idempotency
4. Executes handler
5. Records idempotency key