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/Admin/Billing/RejectPaymentRequestCommandHandlerTests.cs

@@ -0,0 +1,69 @@
+using NSubstitute;
+using PnvPanel.Application.Admin.Billing;
+using PnvPanel.Application.Billing;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Billing;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Admin.Billing;
+
+public class RejectPaymentRequestCommandHandlerTests
+{
+    private readonly ITelegramNotifier _telegramNotifier = Substitute.For<ITelegramNotifier>();
+
+    [Fact]
+    public async Task Handle_WhenAwaitingConfirmation_RejectsAndNotifiesUser()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var adminId = Guid.NewGuid();
+        var userId = Guid.NewGuid();
+        var request = PaymentRequest.Create(userId, PaymentPeriod.Quarter, 1500);
+        request.MarkPaymentSent();
+        dbContext.PaymentRequests.Add(request);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var handler = new RejectPaymentRequestCommandHandler(
+            dbContext,
+            _telegramNotifier,
+            FakeCurrentUser.Authenticated(adminId, "admin")
+        );
+
+        var result = await handler.Handle(
+            new RejectPaymentRequestCommand(request.Id, "Платёж не найден"),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(PaymentRequestStatus.Rejected, request.Status);
+        Assert.Equal("Платёж не найден", request.RejectionReason);
+        await _telegramNotifier
+            .Received(1)
+            .NotifyUserAsync(
+                userId,
+                Arg.Is<string>(m => m.Contains("Платёж не найден")),
+                Arg.Any<string?>(),
+                Arg.Any<CancellationToken>()
+            );
+    }
+
+    [Fact]
+    public async Task Handle_WhenRequestNotFound_ReturnsRequestNotFound()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+
+        var handler = new RejectPaymentRequestCommandHandler(
+            dbContext,
+            _telegramNotifier,
+            FakeCurrentUser.Authenticated(Guid.NewGuid(), "admin")
+        );
+
+        var result = await handler.Handle(
+            new RejectPaymentRequestCommand(Guid.NewGuid(), null),
+            CancellationToken.None
+        );
+
+        Assert.False(result.IsSuccess);
+        Assert.Equal(BillingErrors.RequestNotFound, result.Error);
+    }
+}