Implement extension request and gift functionalities in billing system

- Added new endpoints for creating and managing extension requests, allowing users to request billing period extensions.
- Implemented admin approval processes for extension requests via Telegram, including inline buttons for approval and rejection.
- Introduced a gifting feature for admins to grant additional billing days directly to users without a request.
- Updated the support ticket model to accommodate extension requests and their associated properties.
- Enhanced the Telegram notifier to inform admins of new extension requests and notify users of approval or rejection.
- Updated frontend components to support the new extension request and gifting functionalities, including user interfaces for managing these features.
- Revised API documentation to reflect the new endpoints and their usage in the billing context.

backend/src/PnvPanel.Api/Telegram/PnvBotUpdateHandler.cs

@@ -489,6 +489,55 @@ public sealed class PnvBotUpdateHandler(
 
                 break;
             }
+            case "erq":
+            {
+                if (!await TrySetAdminCurrentUserAsync(services, fromId, cancellationToken))
+                {
+                    await botClient.AnswerCallbackQuery(
+                        callback.Id,
+                        "Недостаточно прав.",
+                        cancellationToken: cancellationToken
+                    );
+                    return;
+                }
+
+                var result =
+                    parts[1] == "approve"
+                        ? await sender.Send(
+                            new ApproveExtensionRequestCommand(requestId),
+                            cancellationToken
+                        )
+                        : await sender.Send(
+                            new RejectExtensionRequestCommand(requestId, Reason: null),
+                            cancellationToken
+                        );
+
+                await botClient.AnswerCallbackQuery(
+                    callback.Id,
+                    result.IsSuccess ? "Готово" : result.Error.Message,
+                    cancellationToken: cancellationToken
+                );
+                if (callback.Message is not null)
+                {
+                    var statusText = result.IsSuccess
+                        ? (
+                            parts[1] == "approve"
+                                ? "✅ Заявка на продление одобрена."
+                                : "❌ Заявка отклонена."
+                        )
+                        : $"⚠️ {result.Error.Message}";
+                    var text = $"{Escape(callback.Message.Text ?? "")}\n\n{statusText}";
+                    await botClient.EditMessageText(
+                        chatId.Value,
+                        callback.Message.Id,
+                        text,
+                        parseMode: ParseMode.Html,
+                        cancellationToken: cancellationToken
+                    );
+                }
+
+                break;
+            }
             case "pay":
             {
                 if (!await TrySetAdminCurrentUserAsync(services, fromId, cancellationToken))