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.Infrastructure/Identity/AppRole.cs

@@ -18,6 +18,10 @@ public class AppRole : IdentityRole<Guid>
 
     public bool IsSystem { get; set; }
 
+    /// <summary>Включает биллинг для пользователей с этой ролью (недоступно для роли admin — см.
+    /// RoleService.UpdateRoleAsync).</summary>
+    public bool BillingEnabled { get; set; }
+
     public AppRole() { }
 
     public AppRole(string name)