Function register_finish 

pub fn register_finish(
    session: AdminSession,
    label: Option<String>,
    credential: Json<RegisterPublicKeyCredential>,
    webauthn: &State<Option<WebauthnState>>,
    store: &State<CeremonyStore>,
    cookies: &CookieJar<'_>,
    pool: &State<DbPool>,
) -> Result<Status, Status>

Enroll, step 2 (POST /admin/passkeys/register/finish?label=): finishes registration with the authenticator’s response, persisting the new passkey (public key only) under an optional label. 400 if the ceremony cookie/state is missing or the attestation doesn’t verify.