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/Common/Interfaces/IAppDbContext.cs

@@ -3,6 +3,7 @@ using Microsoft.EntityFrameworkCore.Infrastructure;
 using PnvPanel.Domain.Activation;
 using PnvPanel.Domain.Apps;
 using PnvPanel.Domain.Audit;
+using PnvPanel.Domain.Billing;
 using PnvPanel.Domain.Configs;
 using PnvPanel.Domain.Inbounds;
 using PnvPanel.Domain.Instructions;
@@ -48,6 +49,10 @@ public interface IAppDbContext
 
     DbSet<PricingSettings> PricingSettings { get; }
 
+    DbSet<BillingSettings> BillingSettings { get; }
+
+    DbSet<PaymentRequest> PaymentRequests { get; }
+
     /// <summary>Нужен для advisory-lock при проверке квоты конфигов (см. CreateVpnConfigCommandHandler).</summary>
     DatabaseFacade Database { get; }