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/Roles/CreateRoleCommandHandler.cs

@@ -15,6 +15,7 @@ public sealed class CreateRoleCommandHandler(IRoleService roleService)
             command.Name,
             command.MaxConfigs,
             command.MaxIpLimit,
+            command.BillingEnabled,
             cancellationToken
         );
 }