✨(backend) add configuration for idle disconnect timeout

Expose idle disconnect timeout as configurable parameter accepting None value
to disable feature entirely, providing emergency killswitch for buggy behavior
without redeployment, following other frontend configuration patterns.

src/backend/meet/settings.py

@@ -326,6 +326,11 @@ class Base(Configuration):
         "is_silent_login_enabled": values.BooleanValue(
             True, environ_name="FRONTEND_IS_SILENT_LOGIN_ENABLED", environ_prefix=None
         ),
+        "idle_disconnect_warning_delay": values.PositiveIntegerValue(
+            None,
+            environ_name="FRONTEND_IDLE_DISCONNECT_WARNING_DELAY",
+            environ_prefix=None,
+        ),
         "feedback": values.DictValue(
             {}, environ_name="FRONTEND_FEEDBACK", environ_prefix=None
         ),