GitLab

- Use proxyutil.ConfigureTransportProxy for unified proxy protocol support
  (HTTP/HTTPS/SOCKS5/SOCKS5H), replacing ad-hoc HTTP-only proxy code
- Proxy errors return ErrProxyUnavailable → gateway triggers account switch
  via UpstreamFailoverError instead of fallback to direct connection
- Timeout: proxy dial 3s, TLS handshake 3s, data transfer 60s
- Mark proxy unavailable for 5 minutes in Redis on connectivity failure
- Quota-weighted load balancing: providers with quota_limit>0 are selected
  by remaining quota (weighted random); quota_limit=0 providers treated as
  0% weight and placed last