feat(admin): add full-chain trace and performance monitoring

- Backend: trace.Logger ring-buffer for request chain events
- Backend: perf.Monitor with runtime.MemStats sampling every 10s
- Backend: /api/v1/admin/trace and /api/v1/admin/perf endpoints
- Frontend: TracePage — timeline with hop icons and status dots
- Frontend: PerformancePage — memory/GC bar charts + live stats
- Admin nav expanded to 11 items

frontend/web/src/components/admin/AdminLayout.tsx

@@ -8,6 +8,8 @@ const navItems = [
   { to: '/admin/plugins', label: '插件管理', icon: '◇' },
   { to: '/admin/memory', label: '记忆管理', icon: '♡' },
   { to: '/admin/bridge', label: '平台桥接', icon: '↗' },
+  { to: '/admin/trace', label: '链路追踪', icon: '▸' },
+  { to: '/admin/perf', label: '性能监控', icon: '▴' },
   { to: '/admin/llm-calls', label: 'LLM 日志', icon: '🧠' },
   { to: '/admin/system', label: '系统信息', icon: '⊞' },
 ]