feat: Telegram Bot Integration

- TelegramService: Non-Streaming Claude Chat mit Tool-Loop,
  Commands (/start, /link, /session, /new, /clear, /status, /unlink),
  Nachrichten-Splitting bei >4096 Zeichen, Typing-Indicator,
  History-Aufbau aus DB, Token-Usage-Tracking
- TelegramEndpoint: Webhook (POST, no-auth), Link-Code-Generierung
  (POST, auth), Webhook-Registrierung + Info (Admin)
- DatabaseService: telegram_links Tabelle mit Link-Code-Flow
  (6-stelliger Code, 5 Min Ablauf), GetUserSessions
- AuthMiddleware: /api/telegram/webhook von Auth ausgenommen
- Session-Verknüpfung via Link-Code (Web-UI generiert Code,
  Telegram /link CODE löst ein)
- TelegramLinkRecord Model

Middleware/AuthMiddleware.cs

@@ -17,8 +17,8 @@ public class AuthMiddleware
     {
         var path = ctx.Request.Path.Value ?? "";
 
-        // Static files and login endpoint don't require auth
-        if (!path.StartsWith("/api/") || path == "/api/auth/login")
+        // Static files, login and telegram webhook don't require auth
+        if (!path.StartsWith("/api/") || path == "/api/auth/login" || path == "/api/telegram/webhook")
         {
             await _next(ctx);
             return;