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

wwwroot/js/pages/chat-page.js

@@ -152,6 +152,36 @@ async function _showKeySwitchModal() {
   var list = document.createElement('div');
   list.className = 'key-switch-list';
 
+  // "Kein Key"-Option als erste Zeile
+  var noneRow = document.createElement('div');
+  noneRow.className = 'key-switch-row';
+  if (!selectedKeyId) noneRow.classList.add('active');
+  var noneInfo = document.createElement('div');
+  noneInfo.className = 'key-switch-info';
+  var noneName = document.createElement('div');
+  noneName.className = 'key-switch-name';
+  noneName.textContent = 'Kein Key';
+  noneInfo.appendChild(noneName);
+  var noneMeta = document.createElement('div');
+  noneMeta.className = 'key-switch-meta';
+  noneMeta.textContent = 'Agent ohne API-Key nutzen';
+  noneInfo.appendChild(noneMeta);
+  noneRow.appendChild(noneInfo);
+  if (selectedKeyId) {
+    noneRow.classList.add('clickable');
+    noneRow.addEventListener('click', async function() {
+      noneRow.classList.add('loading-state');
+      try {
+        await apiSelectApiKey('none', getCurrentAgentId());
+        close();
+        await _initChatUsageBadge();
+      } catch(e) {
+        noneRow.classList.remove('loading-state');
+      }
+    });
+  }
+  list.appendChild(noneRow);
+
   keys.forEach(function(key) {
     var row = document.createElement('div');
     row.className = 'key-switch-row';