Refactor payment request handling to support role change top-ups

- Updated the `PaymentRequest` model to include a new `Kind` property, distinguishing between `Subscription` and `RoleChangeTopUp` requests.
- Modified the `TelegramNotifier` to accommodate the new request type, ensuring accurate notifications for role change top-ups.
- Enhanced the `ConfirmPaymentRequestCommandHandler` to handle role change top-ups without extending the billing period, reflecting the new payment logic.
- Updated various application components and tests to support the new payment request structure and ensure proper functionality.
- Revised API documentation to clarify the behavior of role change top-ups and their impact on billing.

backend/src/PnvPanel.Application/Admin/Billing/AdminPaymentRequestDto.cs

@@ -6,7 +6,8 @@ public sealed record AdminPaymentRequestDto(
     Guid Id,
     Guid UserId,
     string UserName,
-    PaymentPeriod Period,
+    PaymentRequestKind Kind,
+    PaymentPeriod? Period,
     int AmountSnapshot,
     PaymentRequestStatus Status,
     DateTimeOffset CreatedAt