feat(web): M8.6a 交易信号页（/signals）+ 导航与类型

- app/signals/page.tsx：因子+买卖排名阈值+时点+白名单 → BUY/WATCH/SELL 表格
  （类型 Pill 着色、得分/价格/触发理由）+ 最近信号记录回看
- app-shell 导航加入「交易信号」；lib/types.ts 增加 Signal* 类型与 universe.symbols
- 真实 MySQL 端到端：/api/signals 返回可解释信号（跌破 MA60 的 SELL 警示等）；
  /signals 页面 200；tsc --noEmit 通过

frontend/web/components/app-shell.tsx

@@ -19,6 +19,7 @@ const GROUPS: { title: string; items: NavItem[] }[] = [
       { href: "/", label: "总览", icon: "grid" },
       { href: "/stocks", label: "股票池", icon: "candles" },
       { href: "/selection", label: "股票筛选", icon: "target" },
+      { href: "/signals", label: "交易信号", icon: "scale" },
       { href: "/factors", label: "因子研究", icon: "flask" },
       { href: "/factors/compose", label: "因子组合", icon: "layers" },
       { href: "/backtest", label: "选股回测", icon: "gauge" },