Cronjobs + AnthropicService Singleton Refactoring

Cronjobs:
- CronjobService (BackgroundService) prüft jede Minute auf fällige Jobs
- Cron-Expression in Server-Lokalzeit via Cronos (CronFormat.Standard)
- Cronjob-Ausführung: Prompt → Session → Agent-Antwort in Chat-History
- Optionale Benachrichtigungs-Session (notify_session_id): Zusammenfassung
  wird in separate Session geschrieben (z.B. mit Telegram)
- Telegram-Benachrichtigung mit Cronjob-Name + Ursprungs-Session
- REST API: CRUD + toggle + manuelles Ausführen (/run)
- Frontend: Dashboard-Cards, Cronjob-Formular mit Cron-Vorschau,
  Session-Dropdown, Notify-Session-Dropdown, Status-Box

Refactoring:
- AnthropicService als Singleton in DI registriert
- ChatEndpoint + TelegramService nutzen injizierte Instanz
- Redundante Dependencies (Token, ToolService, MemoryService) entfernt

Neue Dateien:
- Models/CronjobRecord.cs
- Services/CronjobService.cs
- Endpoints/CronjobEndpoint.cs
- wwwroot/js/pages/cronjob-page.js
- PLAN-Cronjob.md

Endpoints/ChatEndpoint.cs

@@ -1,7 +1,5 @@
-using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Net.Http;
 using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
@@ -22,7 +20,7 @@ public static class ChatEndpoint
         app.MapPost("/api/chat", HandleChat);
     }
 
-    private static async Task HandleChat(HttpRequest req, HttpResponse res, IHttpClientFactory httpFactory, IConfiguration config, ChatService chatService, DatabaseService db, StreamingTracker streamingTracker, MemoryService memoryService, ToolService toolService, CancellationToken cancellationToken)
+    private static async Task HandleChat(HttpRequest req, HttpResponse res, IConfiguration config, ChatService chatService, DatabaseService db, StreamingTracker streamingTracker, AnthropicService anthropic, CancellationToken cancellationToken)
     {
         var body = await JsonSerializer.DeserializeAsync<ChatRequest>(req.Body);
         if (body == null || body.Messages == null || body.Messages.Count == 0)
@@ -32,14 +30,6 @@ public static class ChatEndpoint
             return;
         }
 
-        var token = config["AnthropicToken"];
-        if (string.IsNullOrEmpty(token))
-        {
-            res.StatusCode = 500;
-            await res.WriteAsJsonAsync(new { error = "AnthropicToken not configured" });
-            return;
-        }
-
         var sessionId = body.SessionId;
         if (string.IsNullOrEmpty(sessionId))
         {
@@ -111,7 +101,6 @@ public static class ChatEndpoint
             }, attachmentRecords);
         }
 
-        var anthropic = new AnthropicService(httpFactory, token, toolService, db, memoryService);
         streamingTracker.Start(sessionId, ctx.ChainId);
         try
         {