POST /auth/login had no rate limiting, no lockout, no delay — a patient
distributed attack could brute-force a password against an enumerable
username list on a custodial wallet, where a guessed password means
withdrawing someone's funds.
Add per-username and per-IP throttling with exponential backoff
(app/auth/rate_limit.py), keyed on app.state like UserLocks rather than
a module global so each app instance gets isolated throttle state.
Unknown-user and wrong-password already shared one response path, so no
enumeration oracle there. Registration is throttled per-IP too, which
also bounds how many accounts one IP can spin up (B-31).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
