Use HTMX-compatible HTML fragments on WebSocket, fix multi-connection

- WebSocket sends HTML fragments with id="price-display" instead of JSON
  for direct HTMX OOB swap on the client
- Switch from ring-undertow-adapter ws wrapper to raw Undertow WebSocket
  API (proxy) to fix single-connection limitation
- Keep JSON endpoint at /api/price/latest unchanged
- Update CLAUDE.md and README.md

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

CLAUDE.md

@@ -44,7 +44,7 @@ Docker runs alongside local dev on different ports and databases:
 ## API Endpoints
 
 - `GET /api/health` — Health check
-- `GET /api/price/ws` — WebSocket endpoint for live JSON price updates
+- `GET /api/price/ws` — WebSocket endpoint for live HTML price fragments (HTMX-compatible)
 - `GET /api/price/latest` — Latest price as JSON
 
 ## Source Layout