Merge WebSocketServer functionality into HttpServer so a single port can
serve both HTTP(S) and WebSocket(S) endpoints while preserving full backward
compatibility for existing HTTP interfaces.
Key changes:
- Add WebSocket route registration: ws(), onWsConnect(), onWsDisconnect()
- Add client management: sendToWsClient(), broadcastWs(), disconnectWsClient()
- Rewrite Session class using beast::tcp_stream for unified HTTP/WS handling
- Support automatic ws:// and wss:// upgrades on HTTP/HTTPS ports
- Thread-safe message queue with asio::post for cross-thread sends
- Update Makefile to link OpenSSL and pthread for TLS/WebSocket support
- Add test_server.cpp demonstrating HTTP + WebSocket mixed usage
