Function heartbeat_callback 

pub async fn heartbeat_callback(
    pool: &PgPool,
    callback_id: Uuid,
    timeout: Duration,
) -> Result<JobRow, AwaError>

Reset the callback timeout for a long-running external operation.

External systems call this periodically to signal “still working” without completing the job. Resets callback_timeout_at to now() + timeout. The job stays in waiting_external.

Returns the updated job row, or CallbackNotFound if the callback ID doesn’t match a waiting job.