fix: suppress noisy gevent SystemExit traceback on shutdown

When stopping gunicorn with Ctrl+C, the gevent worker's handle_quit()
calls sys.exit(0) inside a greenlet, causing gevent to print a
SystemExit traceback. Add a gunicorn config with post_worker_init hook
that marks SystemExit as a non-error in gevent's hub.

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

start.sh

@@ -126,6 +126,7 @@ if [[ "$HAS_GUNICORN" -eq 1 && "$HAS_GEVENT" -eq 1 ]]; then
     echo "[INTERCEPT] Listening on ${HOST}:${PORT}"
 
     GUNICORN_ARGS=(
+        -c "$SCRIPT_DIR/gunicorn.conf.py"
         -k gevent
         -w 1
         --timeout 300