Refactor Telegram bot configuration and deep link handling

- Removed the `BotUsername` property from `TelegramOptions` and updated the `.env.example` to reflect this change, as the bot's username is now dynamically retrieved via the Bot API.
- Introduced `ITelegramBotInfo` to cache the bot's username, improving the handling of deep links in `TelegramEndpoints`.
- Updated API documentation to clarify that the deep link is now dependent on the bot's token and its availability through the Bot API, enhancing clarity for developers.

backend/src/PnvPanel.Api/Program.cs

@@ -85,6 +85,8 @@ builder.Services.AddSingleton<ITelegramBotClient>(sp =>
 });
 // Scoped — зависит от IIdentityService (scoped), не Singleton.
 builder.Services.AddScoped<ITelegramNotifier, TelegramNotifier>();
+// Singleton — кэширует username бота (getMe) на весь процесс, не из ручного env (см. TelegramBotInfo).
+builder.Services.AddSingleton<ITelegramBotInfo, TelegramBotInfo>();
 builder.Services.AddSingleton<PnvBotUpdateHandler>();
 builder.Services.AddHostedService<TelegramBotHostedService>();