v0.7.1: configurable rate limits + timeouts, proxq v0.5.1

- bump proxq v0.4.1 → v0.5.1 (adds Go OpenAI client package, job header tracking)
- all nginx rate limits configurable via env vars:
  RATELIMIT_ADMIN, RATELIMIT_API, RATELIMIT_STORAGE,
  RATELIMIT_CLAUDEBOX, RATELIMIT_BROWSER, RATELIMIT_PROXQ
  plus corresponding _BURST vars for each zone
- all nginx proxy timeouts configurable via env vars:
  TIMEOUT_ADMIN (60s), TIMEOUT_API (600s), TIMEOUT_CLAUDEBOX (600s),
  TIMEOUT_BROWSER (600s), TIMEOUT_PROXQ (30s)
- NGINX_MAX_BODY_SIZE configurable (default 50m)
- README: fix Mistral/Cohere free tier labels in flags table,
  update nginx service description with configurable limits,
  note Redis DB 1 usage by proxq
- .env.example: add rate limit, timeout, and proxq config vars