- Updated `BillingConfigResumer` to manage billing expiration entirely on the application side, using `enable` as the sole control mechanism for client access. - Changed all relevant methods to pass `DateTimeOffset.UnixEpoch` for `expiresAt`, ensuring the panel does not enforce expiration independently of our application logic. - Modified `IXuiPanelGateway` interface to reflect the new expiration handling approach, clarifying the role of `expiresAt` in client management. - Adjusted command handlers for creating and rotating VPN configurations to set `expiresAt` to `null`, preventing unintended expiration enforcement by the panel. - Enhanced documentation to explain the new billing expiration management strategy and its implications for client configurations.