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.

frontend/src/features/billing/PaymentRequestPanel.tsx

@@ -103,8 +103,12 @@ export function PaymentRequestPanel({ status }: { status: BillingStatusDto }) {
       </CardHeader>
       <CardContent className="flex flex-col gap-4">
         <p className="text-sm">
-          {t(`billing.periods.${request.period}`)} — <span className="font-medium">{request.amountSnapshot} ₽</span>
+          {request.kind === 'RoleChangeTopUp' ? t('billing.roleChangeTopUp') : t(`billing.periods.${request.period}`)} —{' '}
+          <span className="font-medium">{request.amountSnapshot} ₽</span>
         </p>
+        {request.kind === 'RoleChangeTopUp' && (
+          <p className="text-xs text-muted-foreground">{t('billing.roleChangeTopUpHint')}</p>
+        )}
 
         {!isAwaitingConfirmation && (
           <div className="flex flex-col gap-1.5">