fix: add graceful-timeout to gunicorn so Ctrl+C shuts down promptly

Long-lived SSE connections prevent the gevent worker from exiting on
SIGINT. --graceful-timeout 5 force-kills the worker after 5 seconds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

start.sh

@@ -127,6 +127,7 @@ if [[ "$HAS_GUNICORN" -eq 1 && "$HAS_GEVENT" -eq 1 ]]; then
         -k gevent
         -w 1
         --timeout 300
+        --graceful-timeout 5
         --worker-connections 1000
         --bind "${HOST}:${PORT}"
         --access-logfile -