feat: 公开用户排行榜页面 + 修复 checkpointer 连接与前端依赖

- 新增公开排行榜页面与后端 public_leaderboard 路由
- 完善 skills/agents 路由
- checkpointer Postgres 改用连接池(自愈), 解决连接关闭后定时任务连环失败
- 修复前端 mermaid 依赖未声明导致 Vite 解析失败 (新增 pnpm-lock)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/App.tsx

@@ -19,6 +19,7 @@ import PublicScheduledTaskPage, {
   PublicScheduledTaskFullscreenPage,
 } from "./pages/PublicScheduledTaskPage";
 import PublicSharePage from "./pages/PublicSharePage";
+import PublicLeaderboardPage from "./pages/PublicLeaderboardPage";
 import PublicRoundtableSharePage from "./pages/PublicRoundtableSharePage";
 import EmbedChatPage from "./pages/EmbedChatPage";
 import { ChatRuntime } from "./pages/WorkspaceRoutes";
@@ -66,6 +67,8 @@ export function App() {
           <Route path="/landing" element={<LandingPage />} />
           {/* Public read-only conversation viewer — no authentication. */}
           <Route path="/share/:shareCode" element={<PublicSharePage />} />
+          {/* Public read-only user leaderboard publish page — no authentication. */}
+          <Route path="/public/leaderboard" element={<PublicLeaderboardPage />} />
           {/* Public read-only roundtable share — landing + full result report. */}
           <Route
             path="/roundtable-share/:shareCode"