Implement rate limiting and enhance authentication flow

- Added rate limiting configuration for authentication endpoints, allowing customizable request limits via environment variables.
- Updated authentication flow to utilize HttpRequest for cookie management, ensuring secure handling of refresh tokens.
- Introduced a new endpoint to retrieve user subscription details.
- Enhanced the handling of Telegram bot token validation to prevent errors with empty tokens.
- Updated the application to serialize enums as strings for better documentation and compatibility with TypeScript.
- Improved test coverage for new features and adjustments in command handlers.

.env.example

@@ -33,6 +33,10 @@ AdminSeed__TelegramUserIds=123456789
 # Квота конфигов для системной роли "user" (выдаётся при регистрации).
 Roles__DefaultUserMaxConfigs=3
 
+# ── Rate limiting ────────────────────────────────────────────────────────
+# Лимит запросов/мин на auth-эндпоинты (login/register/refresh/telegram/subscription). По умолчанию 20.
+# RateLimiting__AuthPermitLimit=20
+
 # ── Telegram-бот ──────────────────────────────────────────────────────────
 # Если BotToken пуст — бот не стартует, панель работает без него.
 Telegram__BotToken=