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/AdminPaymentRequestDto.cs

@@ -0,0 +1,13 @@
+using PnvPanel.Domain.Billing;
+
+namespace PnvPanel.Application.Admin.Billing;
+
+public sealed record AdminPaymentRequestDto(
+    Guid Id,
+    Guid UserId,
+    string UserName,
+    PaymentPeriod Period,
+    int AmountSnapshot,
+    PaymentRequestStatus Status,
+    DateTimeOffset CreatedAt
+);