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/src/PnvPanel.Application/Billing/CreatePaymentRequest/CreatePaymentRequestCommandHandler.cs

@@ -0,0 +1,64 @@
+using Microsoft.EntityFrameworkCore;
+using PnvPanel.Application.Auth;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+using PnvPanel.Domain.Billing;
+
+namespace PnvPanel.Application.Billing.CreatePaymentRequest;
+
+public sealed class CreatePaymentRequestCommandHandler(
+    IAppDbContext dbContext,
+    IIdentityService identityService,
+    ICurrentUser currentUser
+) : ICommandHandler<CreatePaymentRequestCommand, Result<PaymentRequestDto>>
+{
+    public async Task<Result<PaymentRequestDto>> Handle(
+        CreatePaymentRequestCommand command,
+        CancellationToken cancellationToken
+    )
+    {
+        if (currentUser.UserId is not { } userId)
+            return Result.Failure<PaymentRequestDto>(AuthErrors.Unauthorized);
+
+        var profile = await identityService.GetProfileAsync(userId, cancellationToken);
+        if (profile is null)
+            return Result.Failure<PaymentRequestDto>(AuthErrors.Unauthorized);
+
+        if (!profile.BillingEnabled)
+            return Result.Failure<PaymentRequestDto>(BillingErrors.NotEnabled);
+
+        if (profile.MaxConfigs == RoleQuota.Unlimited)
+            return Result.Failure<PaymentRequestDto>(BillingErrors.UnlimitedRoleNotSupported);
+
+        var hasActiveRequest = await dbContext.PaymentRequests.AnyAsync(
+            r =>
+                r.UserId == userId
+                && (
+                    r.Status == PaymentRequestStatus.AwaitingPayment
+                    || r.Status == PaymentRequestStatus.AwaitingConfirmation
+                ),
+            cancellationToken
+        );
+        if (hasActiveRequest)
+            return Result.Failure<PaymentRequestDto>(BillingErrors.ActiveRequestExists);
+
+        var pricing = await dbContext.PricingSettings.AsNoTracking().FirstOrDefaultAsync(cancellationToken);
+        var ratePerMonth = command.Period switch
+        {
+            PaymentPeriod.Quarter => pricing?.PricePerConfigPerQuarter,
+            PaymentPeriod.HalfYear => pricing?.PricePerConfigPerHalfYear,
+            PaymentPeriod.Year => pricing?.PricePerConfigPerYear,
+            _ => null,
+        };
+        if (ratePerMonth is not { } rate)
+            return Result.Failure<PaymentRequestDto>(BillingErrors.PricingNotConfigured);
+
+        var amount = rate * profile.MaxConfigs * command.Period.ToMonths();
+
+        var request = PaymentRequest.Create(userId, command.Period, amount);
+        dbContext.PaymentRequests.Add(request);
+
+        return Result.Success(PaymentRequestDto.FromDomain(request));
+    }
+}