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.Domain.Tests/Billing/PaymentRequestTests.cs

@@ -0,0 +1,101 @@
+using PnvPanel.Domain.Billing;
+using PnvPanel.Domain.Exceptions;
+using Xunit;
+
+namespace PnvPanel.Domain.Tests.Billing;
+
+public class PaymentRequestTests
+{
+    [Fact]
+    public void Create_StartsInAwaitingPayment()
+    {
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Quarter, 1500);
+
+        Assert.Equal(PaymentRequestStatus.AwaitingPayment, request.Status);
+        Assert.Equal(1500, request.AmountSnapshot);
+    }
+
+    [Fact]
+    public void MarkPaymentSent_FromAwaitingPayment_MovesToAwaitingConfirmation()
+    {
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Quarter, 1500);
+
+        request.MarkPaymentSent();
+
+        Assert.Equal(PaymentRequestStatus.AwaitingConfirmation, request.Status);
+    }
+
+    [Fact]
+    public void MarkPaymentSent_WhenAlreadySent_Throws()
+    {
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Quarter, 1500);
+        request.MarkPaymentSent();
+
+        Assert.Throws<DomainException>(() => request.MarkPaymentSent());
+    }
+
+    [Fact]
+    public void Cancel_FromAwaitingPayment_MovesToCancelled()
+    {
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Quarter, 1500);
+
+        request.Cancel();
+
+        Assert.Equal(PaymentRequestStatus.Cancelled, request.Status);
+    }
+
+    [Fact]
+    public void Cancel_AfterMarkPaymentSent_Throws()
+    {
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Quarter, 1500);
+        request.MarkPaymentSent();
+
+        Assert.Throws<DomainException>(() => request.Cancel());
+    }
+
+    [Fact]
+    public void Confirm_FromAwaitingConfirmation_Succeeds()
+    {
+        var adminId = Guid.NewGuid();
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Year, 5000);
+        request.MarkPaymentSent();
+
+        request.Confirm(adminId);
+
+        Assert.Equal(PaymentRequestStatus.Confirmed, request.Status);
+        Assert.Equal(adminId, request.DecidedBy);
+        Assert.NotNull(request.DecidedAt);
+    }
+
+    [Fact]
+    public void Confirm_FromAwaitingPayment_Succeeds()
+    {
+        // Админ мог увидеть оплату раньше, чем пользователь нажал "Я оплатил".
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Year, 5000);
+
+        request.Confirm(Guid.NewGuid());
+
+        Assert.Equal(PaymentRequestStatus.Confirmed, request.Status);
+    }
+
+    [Fact]
+    public void Reject_FromAwaitingConfirmation_SetsReason()
+    {
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.HalfYear, 3000);
+        request.MarkPaymentSent();
+
+        request.Reject(Guid.NewGuid(), "Платёж не найден");
+
+        Assert.Equal(PaymentRequestStatus.Rejected, request.Status);
+        Assert.Equal("Платёж не найден", request.RejectionReason);
+    }
+
+    [Fact]
+    public void Confirm_WhenAlreadyDecided_Throws()
+    {
+        var request = PaymentRequest.Create(Guid.NewGuid(), PaymentPeriod.Quarter, 1500);
+        request.Cancel();
+
+        Assert.Throws<DomainException>(() => request.Confirm(Guid.NewGuid()));
+    }
+}