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>

Services/DatabaseService.cs

@@ -200,6 +200,7 @@ public partial class DatabaseService
         TryAddColumn(conn, "chains", "effort", "TEXT NOT NULL DEFAULT ''");
         TryAddColumn(conn, "sessions", "multi_model_mode", "TEXT");
         TryAddColumn(conn, "sessions", "panel_agent_ids", "TEXT");
+        TryAddColumn(conn, "sessions", "moa_layers", "INTEGER NOT NULL DEFAULT 2");
 
         // 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.