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>

Models/ChatContext.cs

@@ -37,6 +37,7 @@ public class ChatContext
     // der Worker synthetisiert. "off" / "fusion" (später "moa").
     public string MultiModelMode { get; set; }
     public List<SubAgentContext> PanelAgents { get; set; }
+    public int MoaLayers { get; set; } = 2;   // MoA: Anzahl Proposer-Runden (nur bei mode="moa")
 }
 
 public class SubAgentContext