Add graceful shutdown + cancel orders on exit

[Sszmentor]

Problem

No signal handlers. When process receives SIGTERM/SIGINT, WebSocket connections not closed, pending GTC orders remain live on exchange.

Requirements

1. Register SIGTERM/SIGINT handlers in server.py
2. On shutdown: cancel all open orders via cancel_all_orders()
3. Close WebSocket connections gracefully
4. Log shutdown sequence
5. Add configurable CANCEL_ON_SHUTDOWN=true env var

Files

• src/polymarket_mcp/server.py (signal handlers)
• src/polymarket_mcp/auth/client.py:323 (cancel_all_orders exists)
• src/polymarket_mcp/utils/websocket_manager.py (close connections)