TOTP brute-force is a realistic attack once a password is compromised:
with no independent throttle, an attacker shared the login budget (10
attempts) across /login, /register, and /mfa/verify-login, and
/mfa/enable had no rate limiting at all.
- Add a dedicated `mfaAttempts` store so MFA limits are tracked
separately from login attempts
- Introduce `mfaLimiter` (5 attempts / 15 min) applied to both
/mfa/verify-login and /mfa/enable
- Refactor `rateLimiter()` to accept an optional store parameter,
keeping all existing call-sites unchanged
- Include mfaAttempts in the periodic cleanup interval
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