feat: async background chat, performance scaling, and dead code cleanup

Async chat processing eliminates 504 timeouts - POST returns immediately,
backend processes in background, frontend polls for results with timestamps.
Scale to ~12 simultaneous chats via 8 uvicorn workers + 16-thread executor.
Parallelized MCP data collection, 2h session cache, 5min tool timeout.
Full dead code cleanup across backend, frontend, MCP, docker, and nginx.

backend/Dockerfile

@@ -22,4 +22,4 @@ RUN mkdir -p /data
 
 EXPOSE 8000
 
-CMD ["uvicorn", "app:app", "--host", "0.0.0.0", "--port", "8000", "--workers", "2"]
+CMD ["uvicorn", "app:app", "--host", "0.0.0.0", "--port", "8000", "--workers", "8"]