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.

nginx/default.conf

@@ -9,8 +9,6 @@ server {
         index index.html;
         try_files $uri $uri/ /index.html;
         add_header Cache-Control "no-cache, no-store, must-revalidate";
-        add_header Pragma "no-cache";
-        add_header Expires 0;
     }
 
     location /api/ {