Setup walk (byte-exact redirect URI), client-registration essentials
for Authentik / Keycloak / Authelia / Pocket-ID, the security model
(identity pinned to (issuer, sub) after a one-time verified-email link;
SSO sessions never satisfy step-up; native-2FA accounts still get their
second-factor step; MFA policy belongs at the IdP), the OIDC_ONLY
consequences incl. the iOS lockout and the break-glass runbook, and a
troubleshooting table for the recurring failure shapes (issuer
trailing slash, unfollowed discovery redirects, clock skew,
IdP-side redirect_uri mismatch).
