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/routes/admin/billing.tsx

@@ -162,7 +162,9 @@ function RequestsSection() {
               {data.items.map((request) => (
                 <tr key={request.id} className="border-b border-border">
                   <td className="py-2">{request.userName}</td>
-                  <td className="py-2">{t(`billing.periods.${request.period}`)}</td>
+                  <td className="py-2">
+                    {request.kind === 'RoleChangeTopUp' ? t('billing.roleChangeTopUp') : t(`billing.periods.${request.period}`)}
+                  </td>
                   <td className="py-2">{request.amountSnapshot} ₽</td>
                   <td className="py-2">
                     <Badge variant={request.status === 'AwaitingConfirmation' ? 'warning' : 'outline'}>