Split check_site into phases for SSE mode:
1. IPs + routes sent instantly (check event)
2. Each gateway result streamed as it completes (gateway event)
3. Whois streamed when ready (whois event)
4. Throttle results streamed per gateway (throttle event)

This eliminates the 10-18s spinner delay before any results appear.
JSON mode (/api/check without Accept: text/event-stream) unchanged.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>