Provider-Abstraktion: Agents unterstützen jetzt OpenAI + Anthropic

Backend:
- ChatEndpoint: Globaler OpenAI-Fallback entfernt, Key-Check pro Provider
- TelegramService/CronjobService: Provider-Weiche statt hardcoded Anthropic
- AnthropicService: Preparer/Review/Finalizer sind Provider-aware
- OpenAiService: Neue ChatNonStreaming()-Methode für agentic loop
- ToolService: send_to_session lädt Key des Ziel-Agents
- SubAgentContext: Provider-Feld hinzugefügt
- ChatService: BuildSubAgentContext setzt Provider

Frontend:
- Key-Switch-Modal: 'Kein Key'-Option zum Entfernen der Zuweisung
- ApiKeysEndpoint: keyId='none' löscht Key-Zuweisung

Services/CronjobService.cs

@@ -15,6 +15,7 @@ public class CronjobService : BackgroundService
     private readonly DatabaseService _db;
     private readonly ChatService _chatService;
     private readonly AnthropicService _anthropicService;
+    private readonly OpenAiService _openAiService;
     private readonly TelegramService _telegramService;
     private readonly ILogger<CronjobService> _logger;
 
@@ -22,12 +23,14 @@ public class CronjobService : BackgroundService
         DatabaseService db,
         ChatService chatService,
         AnthropicService anthropicService,
+        OpenAiService openAiService,
         TelegramService telegramService,
         ILogger<CronjobService> logger)
     {
         _db = db;
         _chatService = chatService;
         _anthropicService = anthropicService;
+        _openAiService = openAiService;
         _telegramService = telegramService;
         _logger = logger;
     }
@@ -109,8 +112,12 @@ public class CronjobService : BackgroundService
             Content = job.Prompt
         });
 
-        // Non-Streaming Chat ausführen
-        var responseText = await _anthropicService.ChatNonStreaming(ctx, history);
+        // Non-Streaming Chat ausführen – Provider-Weiche
+        string responseText;
+        if (!string.IsNullOrEmpty(ctx.Provider))
+            responseText = await _openAiService.ChatNonStreaming(ctx, history);
+        else
+            responseText = await _anthropicService.ChatNonStreaming(ctx, history);
 
         // Session-Timestamp aktualisieren
         await _db.UpdateSessionTimestamp(job.SessionId);
@@ -172,7 +179,11 @@ public class CronjobService : BackgroundService
                     ["content"] = new List<object> { new { type = "text", text = summary } }
                 }
             };
-            var notifyResponse = await _anthropicService.ChatNonStreaming(notifyCtx, notifyHistory);
+            string notifyResponse;
+            if (!string.IsNullOrEmpty(notifyCtx.Provider))
+                notifyResponse = await _openAiService.ChatNonStreaming(notifyCtx, notifyHistory);
+            else
+                notifyResponse = await _anthropicService.ChatNonStreaming(notifyCtx, notifyHistory);
 
             await _db.UpdateSessionTimestamp(job.NotifySessionId);