Function sign_device_approval 

pub fn sign_device_approval(
    seed_hex: &str,
    node_id: &str,
    target: &str,
    sha256: &str,
    approval_id: &str,
    now_secs: i64,
    now_iso: &str,
) -> Result<SignedApproval, String>

Signs an approval on the owner’s device for exactly one request. The lifetime is the hub’s own (APPROVAL_TTL_SECS); the nonce is fresh.