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/GetBillingSettingsQueryHandler.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+using PnvPanel.Domain.Billing;
+
+namespace PnvPanel.Application.Admin.Billing;
+
+public sealed class GetBillingSettingsQueryHandler(IAppDbContext dbContext)
+    : IQueryHandler<GetBillingSettingsQuery, Result<BillingSettingsDto>>
+{
+    public async Task<Result<BillingSettingsDto>> Handle(
+        GetBillingSettingsQuery query,
+        CancellationToken cancellationToken
+    )
+    {
+        var settings = await dbContext
+            .BillingSettings.AsNoTracking()
+            .FirstOrDefaultAsync(cancellationToken);
+
+        // Ещё не сохранялось ни разу — отдаём дефолты, а не ошибку (см. PricingSettings).
+        return Result.Success(
+            settings is null
+                ? new BillingSettingsDto(string.Empty, BillingSettings.DefaultGraceDays, false)
+                : BillingSettingsDto.FromDomain(settings)
+        );
+    }
+}