Passwordless authentication

Design a passwordless authentication system using passkeys (WebAuthn/FIDO2) for a consumer service with 60M users, replacing passwords to eliminate credential-stuffing and phishing. It must support multi-device sync of passkeys, a phishing-resistant login at 100k logins/sec, and, the hard part, a secure account-recovery path for users who lose all their devices, without reopening a phishing hole. Cover the credential model, the server side of registration/authentication, multi-device, and recovery.