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/components/workspace/settings/appearance-settings-page.tsx

@@ -30,6 +30,10 @@ import {
 } from "@/core/system-settings/hooks";
 import { cn } from "@/lib/utils";
 
+import PrivateSquareSettingsPage from "@/pages/PrivateSquareSettingsPage";
+import PromptPrefixSettingsPage from "@/pages/PromptPrefixSettingsPage";
+import RecommendedQuestionsPage from "@/pages/RecommendedQuestionsPage";
+
 import { SettingsSection } from "./settings-section";
 
 const languageOptions: { value: Locale; label: string }[] = [
@@ -336,6 +340,17 @@ export function AppearanceSettingsPage() {
           </div>
         )}
       </SettingsSection>
+
+      {isAdmin && (
+        <>
+          <Separator />
+          <RecommendedQuestionsPage embedded />
+          <Separator />
+          <PromptPrefixSettingsPage embedded />
+          <Separator />
+          <PrivateSquareSettingsPage embedded />
+        </>
+      )}
     </div>
   );
 }