Add Mixture-of-Agents (MoA) mode on top of Fusion

Extend the panel engine with a layer loop: Fusion is one proposer round,
MoA runs N rounds where each round after the first sees the previous
round's responses via <previous_layer_responses> and refines its answer.
Only the final round is persisted; intermediate rounds are live-only.
Rounds are clamped to 1-5.

- SessionRecord + migration: moa_layers column (default 2)
- RunPanel: layer loop with per-member cloned message injection
- CloneForMember / BuildPreviousLayerBlock helpers for member isolation
- SSE: panel_layer_start / panel_layer_done events
- Frontend: layer_index composite keying, round dividers, MoA option
- Docs: ARCHITECTURE + ARCHITECTURE_QUICK updated

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

Endpoints/ChatEndpoint.cs

@@ -236,11 +236,13 @@ public static class ChatEndpoint
             // danach als Synthesizer über den unveränderten Dispatch unten. Heartbeat läuft bereits,
             // Cancellation (workCt/sseCt) ist verfügbar. Nur bei aktivem Fusion-Modus + nicht-leerem
             // Panel + keiner Choice-Antwort.
-            if (!isChoiceResponse && ctx.MultiModelMode == "fusion" && ctx.PanelAgents != null && ctx.PanelAgents.Count > 0)
+            if (!isChoiceResponse && (ctx.MultiModelMode == "fusion" || ctx.MultiModelMode == "moa") && ctx.PanelAgents != null && ctx.PanelAgents.Count > 0)
             {
                 try
                 {
-                    await anthropic.RunPanel(res, ctx.PanelAgents, messages, ctx.ChainId, sessionId, userApiKey?.ApiKey, workCt, sseCt);
+                    // Fusion = 1 Runde, MoA = MoaLayers Proposer-Runden vor der Synthese.
+                    var panelLayers = ctx.MultiModelMode == "moa" ? System.Math.Max(1, ctx.MoaLayers) : 1;
+                    await anthropic.RunPanel(res, ctx.PanelAgents, messages, ctx.ChainId, sessionId, userApiKey?.ApiKey, panelLayers, workCt, sseCt);
                 }
                 catch (OperationCanceledException) { throw; }
                 catch (Exception ex)