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/Admin/Billing/ListPaymentRequestsQueryHandler.cs

@@ -0,0 +1,50 @@
+using Microsoft.EntityFrameworkCore;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.Admin.Billing;
+
+public sealed class ListPaymentRequestsQueryHandler(
+    IAppDbContext dbContext,
+    IIdentityService identityService
+) : IQueryHandler<ListPaymentRequestsQuery, Result<PagedList<AdminPaymentRequestDto>>>
+{
+    public async Task<Result<PagedList<AdminPaymentRequestDto>>> Handle(
+        ListPaymentRequestsQuery query,
+        CancellationToken cancellationToken
+    )
+    {
+        var page = query.Page <= 0 ? 1 : query.Page;
+        var pageSize = query.PageSize is <= 0 or > 100 ? 20 : query.PageSize;
+
+        var requestsQuery = dbContext.PaymentRequests.AsNoTracking();
+        if (query.StatusFilter is { } status)
+            requestsQuery = requestsQuery.Where(r => r.Status == status);
+
+        var page1 = await requestsQuery
+            .OrderByDescending(r => r.CreatedAt)
+            .ToPagedListAsync(page, pageSize, cancellationToken);
+
+        var userNames = await identityService.GetUserNamesAsync(
+            page1.Items.Select(r => r.UserId).Distinct().ToList(),
+            cancellationToken
+        );
+
+        var items = page1
+            .Items.Select(r => new AdminPaymentRequestDto(
+                r.Id,
+                r.UserId,
+                userNames.GetValueOrDefault(r.UserId, "?"),
+                r.Period,
+                r.AmountSnapshot,
+                r.Status,
+                r.CreatedAt
+            ))
+            .ToList();
+
+        return Result.Success(
+            new PagedList<AdminPaymentRequestDto>(items, page1.Total, page1.Page, page1.PageSize)
+        );
+    }
+}