Implement billing functionality and enhance role management

- Introduced billing capabilities, allowing users to request payments for subscription periods (3/6/12 months) with admin approval via Telegram.
- Updated role management to include a `BillingEnabled` property, preventing billing for admin roles.
- Enhanced the `CreateRoleCommand` and `UpdateRoleCommand` to accept billing parameters, ensuring proper handling during role creation and updates.
- Added new endpoints for billing management and integrated billing checks into VPN config creation to enforce payment requirements.
- Updated related services, models, and tests to support the new billing features, ensuring comprehensive coverage and functionality.
- Enhanced documentation to reflect the new billing processes and role management changes.

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

@@ -0,0 +1,75 @@
+using NSubstitute;
+using PnvPanel.Application.Billing;
+using PnvPanel.Application.Billing.MarkPaymentSent;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Billing;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Billing.MarkPaymentSent;
+
+public class MarkPaymentSentCommandHandlerTests
+{
+    private readonly IIdentityService _identityService = Substitute.For<IIdentityService>();
+    private readonly ITelegramNotifier _telegramNotifier = Substitute.For<ITelegramNotifier>();
+
+    [Fact]
+    public async Task Handle_WhenAwaitingPayment_MovesToAwaitingConfirmationAndNotifiesAdmins()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var request = PaymentRequest.Create(userId, PaymentPeriod.Year, 6000);
+        dbContext.PaymentRequests.Add(request);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var handler = new MarkPaymentSentCommandHandler(
+            dbContext,
+            _identityService,
+            _telegramNotifier,
+            FakeCurrentUser.Authenticated(userId, "alice")
+        );
+
+        var result = await handler.Handle(
+            new MarkPaymentSentCommand(request.Id),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(PaymentRequestStatus.AwaitingConfirmation, request.Status);
+        await _telegramNotifier
+            .Received(1)
+            .NotifyAdminsPaymentRequestedAsync(
+                request.Id,
+                Arg.Any<string>(),
+                PaymentPeriod.Year,
+                6000,
+                Arg.Any<CancellationToken>()
+            );
+    }
+
+    [Fact]
+    public async Task Handle_WhenAlreadyAwaitingConfirmation_ReturnsNotAwaitingPayment()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var request = PaymentRequest.Create(userId, PaymentPeriod.Year, 6000);
+        request.MarkPaymentSent();
+        dbContext.PaymentRequests.Add(request);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var handler = new MarkPaymentSentCommandHandler(
+            dbContext,
+            _identityService,
+            _telegramNotifier,
+            FakeCurrentUser.Authenticated(userId)
+        );
+
+        var result = await handler.Handle(
+            new MarkPaymentSentCommand(request.Id),
+            CancellationToken.None
+        );
+
+        Assert.False(result.IsSuccess);
+        Assert.Equal(BillingErrors.RequestNotAwaitingPayment, result.Error);
+    }
+}