SSE sync stream (continuous push)

Server-Sent Events endpoint for continuous sync. Streams sync events in real-time after an initial catch-up phase.

Protocol:

1. Client opens SSE connection with JWT token and cursor
2. Server sends all events since cursor as event: sync messages
3. Server sends event: caught_up when historical events are exhausted
4. Server streams new events as they occur via Redis pub/sub
5. Heartbeat every 25s: event: heartbeat

SSE event types:

• sync — sync event data (same format as polling endpoint)
• caught_up — initial catch-up complete, now in real-time mode
• heartbeat — keep-alive signal

Requires SSE support (EventSource API in browsers, or SSE client library). Falls back to polling endpoint (GET /api/im/sync) if SSE unavailable.