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/Support/ApproveExtensionRequestCommandHandlerTests.cs

@@ -144,6 +144,20 @@ public class ApproveExtensionRequestCommandHandlerTests
         Assert.True(result.IsSuccess);
         Assert.Equal(ConfigStatus.Active, config.Status);
         Assert.NotNull(config.ExpiresAt);
+        // enable:true — истечение управляется полностью на нашей стороне, панельный expiresAt
+        // сбрасывается в "без срока" (см. BillingConfigResumer.NoExpiry), а не в реальный новый срок.
+        await _gateway
+            .Received(1)
+            .UpdateClientAsync(
+                Arg.Any<Node>(),
+                inbound.RemoteInboundId,
+                "ext-1",
+                VpnProtocol.Vless,
+                Arg.Any<string>(),
+                true,
+                Arg.Is<DateTimeOffset?>(d => d == DateTimeOffset.UnixEpoch),
+                Arg.Any<CancellationToken>()
+            );
     }
 
     [Fact]