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.Infrastructure/Telegram/TelegramOptions.cs

@@ -5,7 +5,6 @@ public sealed class TelegramOptions
     public const string SectionName = "Telegram";
 
     public string? BotToken { get; init; }
-    public string? BotUsername { get; init; }
     public string PublicSiteUrl { get; init; } = string.Empty;
 
     /// <summary>Прокси для запросов к Bot API (обычно socks5://[user:pass@]host:port). Пусто — без прокси.</summary>