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

Endpoints/ApiKeysEndpoint.cs

@@ -524,6 +524,22 @@ public static class ApiKeysEndpoint
         var userId = ctx.GetUserId();
         if (userId == null) { ctx.Response.StatusCode = 401; return; }
 
+        var agentId = ctx.Request.Query["agentId"].FirstOrDefault();
+        if (string.IsNullOrEmpty(agentId))
+        {
+            ctx.Response.StatusCode = 400;
+            await ctx.Response.WriteAsJsonAsync(new { error = "agentId required" });
+            return;
+        }
+
+        // "none" = Key-Zuweisung entfernen
+        if (keyId == "none")
+        {
+            await db.SetAgentApiKey(userId, agentId, null);
+            await ctx.Response.WriteAsJsonAsync(new { ok = true, keyId = (string)null, agentId, name = "Kein Key" });
+            return;
+        }
+
         var key = await db.GetApiKey(keyId);
         if (key == null)
         {
@@ -540,14 +556,6 @@ public static class ApiKeysEndpoint
             return;
         }
 
-        var agentId = ctx.Request.Query["agentId"].FirstOrDefault();
-        if (string.IsNullOrEmpty(agentId))
-        {
-            ctx.Response.StatusCode = 400;
-            await ctx.Response.WriteAsJsonAsync(new { error = "agentId required" });
-            return;
-        }
-
         await db.SetAgentApiKey(userId, agentId, keyId);
         await ctx.Response.WriteAsJsonAsync(new { ok = true, keyId, agentId, name = key.Name });
     }