fix(wsgi): only install SIGCHLD handler from the main thread

runserver imports the WSGI application from a worker thread, where
signal.signal() raises ValueError and the dev server fails to start.
Gunicorn imports it from the worker's main thread, so production
keeps reaping child processes as before.

webvirtcloud/wsgi.py

@@ -8,7 +8,12 @@ https://docs.djangoproject.com/en/4.2/howto/deployment/wsgi/
 """
 
 import signal
-signal.signal(signal.SIGCHLD, signal.SIG_IGN)
+import threading
+
+# Reap child processes to avoid zombies. Signal handlers can only be set from
+# the main thread; runserver imports this module from a worker thread.
+if threading.current_thread() is threading.main_thread():
+    signal.signal(signal.SIGCHLD, signal.SIG_IGN)
 
 import os