Clear the cross-network verification attempt lockout for a card,
identified by its cardId. This re-enables verification attempts for the
card after it has been throttled by too many recent hard failures — the
account-scoped, cross-network throttle that protects the non-HIGHEST tiers
(Visa/Stripe, Mastercard/TNS). Idempotent: unlocking a card that has no
active lock succeeds and clears nothing. The HIGHEST per-fingerprint lockout
is not affected by this endpoint.