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>

Services/DatabaseService.cs

@@ -198,6 +198,8 @@ public partial class DatabaseService
         TryAddColumn(conn, "messages", "tag", "TEXT");
         TryAddColumn(conn, "agents", "effort", "TEXT NOT NULL DEFAULT ''");
         TryAddColumn(conn, "chains", "effort", "TEXT NOT NULL DEFAULT ''");
+        TryAddColumn(conn, "sessions", "multi_model_mode", "TEXT");
+        TryAddColumn(conn, "sessions", "panel_agent_ids", "TEXT");
 
         // Migration: 1M-Suffix entfernen (Mai 2026). Seit März 2026 ist 1M-Context Default
         // bei Opus 4.6/4.7/4.8 + Sonnet 4.6 – kein "[1m]"-Suffix mehr nötig.