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/package.json

@@ -74,6 +74,7 @@
     "hast": "^1.0.0",
     "katex": "^0.16.28",
     "lucide-react": "^0.562.0",
+    "mermaid": "11.14.0",
     "motion": "^12.26.2",
     "nanoid": "^5.1.6",
     "next-themes": "^0.4.6",