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/Admin/Billing/RejectPaymentRequestCommandHandlerTests.cs

@@ -135,6 +135,8 @@ public class RejectPaymentRequestCommandHandlerTests
 
         Assert.True(result.IsSuccess);
         Assert.Equal(ConfigStatus.Expired, config.Status);
+        // enable:false — истечение управляется полностью на нашей стороне, панельный expiresAt
+        // сбрасывается в "без срока" (см. BillingConfigResumer.NoExpiry), не реальную просроченную дату.
         await _gateway
             .Received(1)
             .UpdateClientAsync(
@@ -144,7 +146,7 @@ public class RejectPaymentRequestCommandHandlerTests
                 VpnProtocol.Vless,
                 Arg.Any<string>(),
                 false,
-                Arg.Any<DateTimeOffset?>(),
+                Arg.Is<DateTimeOffset?>(d => d == DateTimeOffset.UnixEpoch),
                 Arg.Any<CancellationToken>()
             );
         // Фронт (/billing) должен узнать о смене статуса сразу, не дожидаясь опроса.