feat: 用户端访问足迹统计（IP 去重 UV/PV）

- 后端 Visit 模型(date+ip 唯一索引)：POST /api/track 埋点(ClientIP upsert，pv+1)，GET /api/admin/visits 每日 UV/PV 统计
- web 每会话上报一次访问(sessionStorage 防重)，IP 由后端去重
- admin 访问统计页：侧边栏入口 + 今日/近N天 UV·PV 卡片 + 每日柱状趋势(自绘无依赖) + 7/30/90 天切换

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

admin/src/components/admin-sidebar.tsx

@@ -1,5 +1,13 @@
 import { Link, NavLink, useNavigate } from 'react-router-dom'
-import { FileText, LayoutDashboard, LogOut, Moon, Sun, X } from 'lucide-react'
+import {
+  BarChart3,
+  FileText,
+  LayoutDashboard,
+  LogOut,
+  Moon,
+  Sun,
+  X,
+} from 'lucide-react'
 import { logout } from '@/api/auth'
 import { useTheme } from '@/components/theme-provider'
 import { LogoMark } from '@/components/logo'
@@ -8,6 +16,7 @@ import { cn } from '@/lib/utils'
 const NAV = [
   { to: '/', label: '仪表盘', icon: LayoutDashboard, end: true },
   { to: '/posts', label: '文章', icon: FileText, end: false },
+  { to: '/visits', label: '访问统计', icon: BarChart3, end: false },
 ]
 
 interface Props {