Function send_event 

pub async fn send_event(
    client: &Client,
    url: &str,
    delivery_id: &str,
    event_type: &str,
    secret: Option<&str>,
    body: &Value,
) -> SendResult

POST a signed webhook body. The body is serialized ONCE and both signed and sent verbatim so the X-Heldar-Signature always covers the exact bytes the receiver gets. Used by the delivery loop and by the synthetic /test route.