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/Billing/CreatePaymentRequest/CreatePaymentRequestCommandHandler.cs

@@ -32,9 +32,12 @@ public sealed class CreatePaymentRequestCommandHandler(
         if (profile.MaxConfigs == RoleQuota.Unlimited)
             return Result.Failure<PaymentRequestDto>(BillingErrors.UnlimitedRoleNotSupported);
 
+        // RoleChangeTopUp не считается активной подписной заявкой — доплата за смену роли не должна
+        // мешать пользователю продлить/оформить обычную подписку.
         var hasActiveRequest = await dbContext.PaymentRequests.AnyAsync(
             r =>
                 r.UserId == userId
+                && r.Kind == PaymentRequestKind.Subscription
                 && (
                     r.Status == PaymentRequestStatus.AwaitingPayment
                     || r.Status == PaymentRequestStatus.AwaitingConfirmation