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.

docker-compose.yml

@@ -12,8 +12,6 @@ services:
       - OCI_CLI_SUPPRESS_FILE_PERMISSIONS_WARNING=True
     volumes:
       - agent-data:/data
-    ports:
-      - "8000:8000"    # Optional: direct API access for debugging
     networks:
       - agent-net
     healthcheck: