When any client pushes changes, the server broadcasts an SSE event to
all connected clients watching that group. Clients immediately pull
the latest data — no polling delay, no page reload.
Server (~30 lines):
- GET /events?group=G — SSE endpoint, holds connection open
- On successful POST /sync, notify all SSE listeners for affected groups
- Auth-aware: respects token and group permissions
- Auto-cleanup on disconnect via http-kit on-close
Client (~25 lines):
- EventSource connects to /events?group=G on sync loop start
- On SSE message, triggers do-pull! to fetch latest data
- Auto-reconnects (browser EventSource built-in behavior)
- Cleanup on destroy!
Periodic polling remains as fallback (30s default). SSE provides
sub-second sync for the common case.
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