Refactor billing configuration management to centralize expiration handling

- 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.

backend/tests/PnvPanel.Application.Tests/Billing/MarkPaymentSent/MarkPaymentSentCommandHandlerTests.cs

@@ -127,7 +127,8 @@ public class MarkPaymentSentCommandHandlerTests
             .Handle(new MarkPaymentSentCommand(request.Id), CancellationToken.None);
 
         Assert.True(result.IsSuccess);
-        // enable:true + expiresAt в будущем — держим клиента рабочим на панели, пока заявка на проверке.
+        // enable:true — держим клиента рабочим на панели, пока заявка на проверке; expiresAt всегда
+        // сбрасывается в "без срока" (см. BillingConfigResumer.NoExpiry), истечение — только локально.
         await _gateway
             .Received(1)
             .UpdateClientAsync(
@@ -137,7 +138,7 @@ public class MarkPaymentSentCommandHandlerTests
                 VpnProtocol.Vless,
                 Arg.Any<string>(),
                 true,
-                Arg.Is<DateTimeOffset?>(d => d > DateTimeOffset.UtcNow),
+                Arg.Is<DateTimeOffset?>(d => d == DateTimeOffset.UnixEpoch),
                 Arg.Any<CancellationToken>()
             );
         await _notifier.Received(1).NotifyBillingStatusChangedAsync(userId, Arg.Any<CancellationToken>());