chore: set antigravity fallback cooldown default to 1

backend/internal/config/config.go

@@ -772,7 +772,7 @@ func setDefaults() {
 	viper.SetDefault("gateway.failover_on_400", false)
 	viper.SetDefault("gateway.max_account_switches", 10)
 	viper.SetDefault("gateway.max_account_switches_gemini", 3)
-	viper.SetDefault("gateway.antigravity_fallback_cooldown_minutes", 5)
+	viper.SetDefault("gateway.antigravity_fallback_cooldown_minutes", 1)
 	viper.SetDefault("gateway.max_body_size", int64(100*1024*1024))
 	viper.SetDefault("gateway.connection_pool_isolation", ConnectionPoolIsolationAccountProxy)
 	// HTTP 上游连接池配置（针对 5000+ 并发用户优化）