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.Domain/Billing/PaymentRequestKind.cs

@@ -0,0 +1,10 @@
+namespace PnvPanel.Domain.Billing;
+
+/// <summary>Subscription — обычная оплата за период (Period задан). RoleChangeTopUp — доплата разницы
+/// в цене при апгрейде роли с активным оплаченным периодом (Period не задан, не продлевает
+/// BillingPaidUntil) — см. RoleChangeTopUp.Compute.</summary>
+public enum PaymentRequestKind
+{
+    Subscription,
+    RoleChangeTopUp,
+}