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>

.gitignore

@@ -1,7 +1,7 @@
 # Workspace/editor
 .vscode/
 frontend-web/.vscode/
-
+node_modules/
 # Claude Code local-only settings (per-user permissions, not shared)
 .claude/settings.local.json
 
@@ -84,6 +84,9 @@ offline-backend-20260512/backend/tests/*
 deploy
 .history
 
+# Cloned third-party repos (local only)
+git-clone/
+
 # Repo-root offline dependency pack (local only)
 /dist/