feat: replace console HTTP polling with persistent SSE streams (#266)

* feat: replace console HTTP polling with persistent SSE streams

Console collector now subscribes to each server node's /v1/api/events/global
SSE stream for real-time state updates instead of polling /v1/api/dashboard
and /health every 15 seconds.

Server changes:
- Emit ws_created/ws_closed events on global queue from create/close handlers
- Add node_snapshot on SSE connect (workstreams, health, aggregate)
- Add ?expected_node_id= identity verification (409 on mismatch)
- Add health_changed callback to BackendHealthMonitor circuit breaker
- Add periodic aggregate emitter thread (10s)

Console collector changes:
- Single asyncio event loop on one thread multiplexes all SSE connections
  (scales to 1000+ nodes vs thread-per-node)
- Discovery loop spawns/cancels async SSE tasks per node
- Snapshot reconciliation on connect, delta application for live events
- Fix ws_state→cluster_state event type mismatch
- Remove polling code (poll_interval, max_poll_workers, --poll-interval CLI)

SDK changes:
- Add NodeSnapshotEvent, HealthChangedEvent, AggregateEvent dataclasses
- Add stream_node_events() method (async + sync)

* fix: address review feedback on node event streams

- Fix stop() to let SSE manager exit naturally instead of force-stopping
  the event loop (ensures finally cleanup runs)
- Guard against empty/invalid SSE data from ping frames
- Treat missing node_id as identity mismatch (409) when expected_node_id
  is provided
- Fix stale docstring on _update_metrics

docs/docker.md

@@ -69,7 +69,6 @@ All configuration is via environment variables in `.env` (copy from `.env.exampl
 | Variable | Default | Description |
 |----------|---------|-------------|
 | `CONSOLE_PORT` | `8090` | Host port mapping |
-| `CONSOLE_POLL_INTERVAL` | `10` | Node polling interval (seconds) |
 
 ### Auth