Function check_assertion 

pub async fn check_assertion(
    db: &Db,
    c: &impl GenericClient,
    rp: &Rp,
    purpose: &str,
    a: &AssertionJson,
    want: Option<i64>,
) -> Result<(i64, i64)>

Check a sign-in against the challenge it answers: the passkey’s id and player. want limits it to one player’s passkeys (the staff check).