fix(server): register SIGINT handler for graceful shutdown (#7078)

ctl.sh start spawns the server from a non-interactive bash background job,
which causes the child to inherit SIGINT=SIG_IGN. The Stop server buttonsends SIGINT (os.kill(pid, signal.SIGINT)), which is silently ignored.

Register _request_shutdown for SIGINT in addition to SIGTERM so the
shutdown button works for ctl.sh-managed daemons.

server.py

@@ -720,6 +720,11 @@ def main() -> None:
         # Not on the main thread (e.g. embedded/test harness); skip handler.
         logger.debug("Could not install SIGTERM handler", exc_info=True)
 
+    try:
+        signal.signal(signal.SIGINT, _request_shutdown)
+    except (ValueError, OSError):
+        logger.debug("Could not install SIGINT handler", exc_info=True)
+
     try:
         httpd.serve_forever()
     finally: