Bump to v2.3.0: replace polling with SSE stream, fix detector imports

- Add GET /stream SSE endpoint to aggregator.py; state is pushed
  instantly on every change instead of fetched every 2s
- Replace setInterval polling in index.html with EventSource;
  onerror shows the ( ?.?) face and auto-reconnect is handled by
  the browser natively
- Fix ModuleNotFoundError in detectors: inject project root into
  PYTHONPATH when launching subprocesses from kao.py
- Update openapi.yaml, CLAUDE.md, README.md with /stream endpoint
- Remove completed SSE item from TODO.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TODO.md

@@ -4,8 +4,6 @@ Feature ideas for future work, roughly in priority order.
 
 ## REST API improvements
 
-- **`/status` SSE stream** — replace frontend polling with a Server-Sent Events
-  endpoint so the display reacts instantly and the 2s polling overhead disappears
 - **Notification queue** — buffer rapid `/notify` calls and auto-advance through
   them instead of clobbering; important when HA fires several events at once
 - **Sticky notifications** — a `sticky: true` flag on `/notify` to keep a