feat: board-first orchestration with Gateway Bridge, live-update, and flow-board

- GatewayBridgeController: MCP-artiger Kommando-Adapter für Agent-zu-Backend
- TaskBridgeService + LiveUpdateService: SSE Live-Sync + Bridge-Kommandos
- FlowBoard.vue: Board-first orchestration dashboard panel
- live-sync.ts store + live.ts service: SSE-basierte Live-Updates
- Nullability-Warnung in HealthController.cs gefixt
- nginx.conf: SSE-Proxy + CORS für Bridge-Endpunkte
- .gitignore: pnpm/corepack local caches ausgeschlossen
- docs: architecture-board-first-orchestration.md hinzugefügt
- README: Backend Bridge API dokumentiert

backend/Controllers/HealthController.cs

@@ -40,14 +40,14 @@ public class HealthController(IAgentRuntime runtime, HealthCheckService healthCh
             {
                 status = e.Value.Status.ToString(),
                 description = e.Value.Description,
-                data = e.Value.Data
+                data = (IReadOnlyDictionary<string, object?>)e.Value.Data
             });
 
         entries["runtime"] = new
         {
             status = runtimeStatus,
-            description = runtimeDetail ?? "Runtime status checked",
-            data = (IReadOnlyDictionary<string, object>)new Dictionary<string, object>()
+            description = runtimeDetail,
+            data = (IReadOnlyDictionary<string, object?>)new Dictionary<string, object?>()
         };
 
         var isHealthy = report.Status == HealthStatus.Healthy && runtimeStatus == "Online";