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/SessionEndpoint.cs

@@ -148,8 +148,9 @@ public static class SessionEndpoint
         var preparerAgentId = body.TryGetProperty("preparerAgentId", out var pra) ? pra.GetString() : null;
         var multiModelMode = body.TryGetProperty("multiModelMode", out var mmm) ? mmm.GetString() : null;
         var panelAgentIds = body.TryGetProperty("panelAgentIds", out var pai) && pai.ValueKind != JsonValueKind.Null ? pai.GetString() : null;
+        var moaLayers = body.TryGetProperty("moaLayers", out var mol) && mol.ValueKind == JsonValueKind.Number ? mol.GetInt32() : 2;
 
-        var session = await db.CreateSession(userId, name, description, image, imageSmall, agentId, workspace, context, customCommands, avatarScale, avatarOffsetX, avatarOffsetY, reviewAgentId, maxReviewIterations, finalizerAgentId, preparerAgentId, multiModelMode, panelAgentIds);
+        var session = await db.CreateSession(userId, name, description, image, imageSmall, agentId, workspace, context, customCommands, avatarScale, avatarOffsetX, avatarOffsetY, reviewAgentId, maxReviewIterations, finalizerAgentId, preparerAgentId, multiModelMode, panelAgentIds, moaLayers);
         await ctx.Response.WriteAsJsonAsync(session);
     }