sign in as user

Sign in a regular user account, this only works for accounts with the setting "allow_login_as_user" enabled

Mints a session token for the account in user_guid WITHOUT any password check (server-side impersonation). It succeeds ONLY when the target account itself carries the setting allow_login_as_user = on; the caller's own privileges are never consulted. That flag is isolation-restricted: the platform refuses to save it together with any other enabled setting except allow_passwordless_login, so admin/root or otherwise privileged accounts can never be signed in as : by design. Blueprints may grant the flag freely only when CREATING the account; enabling it on an existing account requires a caller holding setting_manage_users. On failure success=false, auth/accesstoken/user stay unset (downstream readers get empty defaults), and error names the reason: USER_IS_NOT_ENABLED_FOR_SIGNIN_AS (target lacks the flag : the usual cause), USER_NOT_FOUND, MISSING_USER_GUID, AUTH_SERVICE_NOT_AVAILABLE, or the auth service's own error verbatim. Always read the error pin before debugging a downstream 'no auth' symptom. A target with 2FA gets a token that starts unverified.