Restricts one suspicious account as a precaution: its risk state becomes QUARANTINED and its evaluations get more friction (verifications or review instead of a direct denied). Only allowed from NORMAL: an account that is BLOCKED or RECOVERING cannot be quarantined and returns 409. The only way out of QUARANTINED is a recovery.

Safe to retry: if the account is already in that state, it returns 200 with the existing resource (the same id), without side effects or a new history entry. A retry with different data (for example, another reason) does not change the account or the recorded reason.