feat: add rate limit bypass for IPs and tokens
Replace tower_governor with custom middleware using governor crate
directly. Adds GATEKEEPER_RATE_LIMIT_BYPASS_KEY (x-ratelimit-bypass
header) and GATEKEEPER_RATE_LIMIT_BYPASS_IPS (comma-separated) env
vars to exempt specific requests from rate limiting.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>