Add multi-model panel with Fusion mode

Panel agents answer the user prompt in parallel (read-only, non-streaming);
their responses are merged into the worker's last user message as a
<panel_responses> block, and the session agent synthesizes one final
streamed answer. The RunPanel engine is built layer-ready for future MoA.

- New session fields: multi_model_mode (off/fusion), panel_agent_ids (CSV)
  with idempotent column migration
- New partial Services/AnthropicService.Panel.cs holding the RunPanel engine
- Provider-agnostic members: Anthropic RunNonStreamingToolLoop / OpenAI
  CompleteText / CLI plan-mode; all forced read-only, persisted with Tag=panel
- SSE events: panel_start, panel_member_start, panel_member_done, panel_done
- Frontend: multi-agent picker, live panel bubbles, history rendering
- Graceful degradation: panel errors fall back to worker-only

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wwwroot/js/chat.js

@@ -128,6 +128,23 @@ function handleStreamEvent(eventType, data) {
       finalizeReviewBubble('done', data, 'finalizer');
       break;
 
+    // Multi-Modell-Panel (Fusion): mehrere Member parallel, non-streaming
+    case 'panel_start':
+      startPanel(data);
+      break;
+
+    case 'panel_member_start':
+      addPanelMemberBubble(data);
+      break;
+
+    case 'panel_member_done':
+      finalizePanelMember(data);
+      break;
+
+    case 'panel_done':
+      endPanel(data);
+      break;
+
     case 'auto_context':
       addAutoContextBubble(data.context);
       break;