Introduce a Redis-backed LoginGuard that combines per-IP, per-identifier
and IP+identifier rate limits, exponential lockout on consecutive failures,
and a concurrent-attempt lock so a single subject cannot pile up parallel
brute-force tries. The guard ships with an in-memory fallback when Redis
is unreachable so login protection keeps working under degraded mode.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
