Enables public beta signup metering: DAILY_OPEN_SIGNUPS env var controls
how many users can register without an invite code per day (0=disabled,
-1=unlimited, N=daily cap). Invite codes always bypass the limit.
Also adds per-IP signup throttling (DAILY_SIGNUPS_PER_IP, default 3/day)
and fail-closed rate limiting on auth endpoints when Redis is down.
Client dynamically fetches /api/auth/signup-info to show invite field
as optional with remaining slots when open signups are enabled.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
