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/UpdateBillingSettingsCommandValidator.cs

@@ -0,0 +1,13 @@
+using FluentValidation;
+
+namespace PnvPanel.Application.Admin.Billing;
+
+public sealed class UpdateBillingSettingsCommandValidator
+    : AbstractValidator<UpdateBillingSettingsCommand>
+{
+    public UpdateBillingSettingsCommandValidator()
+    {
+        RuleFor(x => x.RequisitesText).NotEmpty().MaximumLength(4000);
+        RuleFor(x => x.GraceDays).GreaterThanOrEqualTo(0).LessThanOrEqualTo(365);
+    }
+}