fix(config): register proxy probe URLs default

backend/internal/config/config.go

@@ -2513,6 +2513,7 @@ func setEnvReachableDefaults() {
 	viper.SetDefault("gateway.session_idle_timeout_minutes", 0)
 	viper.SetDefault("gateway.user_message_queue.mode", "")
 	viper.SetDefault("update.proxy_url", "")
+	viper.SetDefault("security.proxy_probe.urls", []ProbeURLConfig{})
 
 	// sticky_escape_enabled is the one exception to the zero-value rule: its
 	// effective default is true, applied post-unmarshal via a viper.IsSet guard.