feat(admin): Admin V2 — Ant Design Pro 纯 SPA 重写

Next.js SSR/hydration 与 SWR fetch-on-mount 存在根本冲突：
hydration 卸载组件时 abort 的请求仍占用后端 DB 连接，
retry 循环耗尽 PostgreSQL 连接池导致后端完全卡死。

admin-v2 使用 Vite + React + antd 纯 SPA 彻底消除此问题：
- 12 页面全部完成（Login, Dashboard, Accounts, Providers, Models,
  API Keys, Usage, Relay, Config, Prompts, Logs, Agent Templates）
- ProTable + ProForm + ProLayout 统一 UI 模式
- TanStack Query + Axios + Zustand 数据层
- JWT 自动刷新 + 401 重试机制
- 全部 18 网络请求 200 OK，零 ERR_ABORTED

同时更新 troubleshooting 第 13 节和 SaaS 平台文档。

admin-v2/src/pages/Usage.tsx

@@ -0,0 +1,120 @@
+// ============================================================
+// 用量统计
+// ============================================================
+
+import { useState } from 'react'
+import { useQuery } from '@tanstack/react-query'
+import { Card, Row, Col, Select, Spin, Alert, Statistic, Typography } from 'antd'
+import { ColumnWidthOutlined, ThunderboltOutlined } from '@ant-design/icons'
+import type { ProColumns } from '@ant-design/pro-components'
+import { ProTable } from '@ant-design/pro-components'
+import { usageService } from '@/services/usage'
+import { telemetryService } from '@/services/telemetry'
+import type { DailyUsageStat, ModelUsageStat } from '@/types'
+
+const { Title } = Typography
+
+export default function Usage() {
+  const [days, setDays] = useState(30)
+
+  const { data: dailyData, isLoading: dailyLoading, error: dailyError } = useQuery({
+    queryKey: ['usage-daily', days],
+    queryFn: () => telemetryService.dailyStats({ days }),
+  })
+
+  const { data: modelData, isLoading: modelLoading } = useQuery({
+    queryKey: ['usage-model', days],
+    queryFn: () => telemetryService.modelStats({}),
+  })
+
+  if (dailyError) {
+    return <Alert type="error" message="加载用量数据失败" description={(dailyError as Error).message} showIcon />
+  }
+
+  const totalRequests = dailyData?.reduce((s, d) => s + d.request_count, 0) ?? 0
+  const totalTokens = dailyData?.reduce((s, d) => s + d.input_tokens + d.output_tokens, 0) ?? 0
+
+  const dailyColumns: ProColumns<DailyUsageStat>[] = [
+    { title: '日期', dataIndex: 'day', width: 120 },
+    { title: '请求数', dataIndex: 'request_count', width: 100, render: (_, r) => r.request_count.toLocaleString() },
+    { title: '输入 Token', dataIndex: 'input_tokens', width: 120, render: (_, r) => r.input_tokens.toLocaleString() },
+    { title: '输出 Token', dataIndex: 'output_tokens', width: 120, render: (_, r) => r.output_tokens.toLocaleString() },
+    { title: '设备数', dataIndex: 'unique_devices', width: 80 },
+  ]
+
+  const modelColumns: ProColumns<ModelUsageStat>[] = [
+    { title: '模型', dataIndex: 'model_id', width: 200 },
+    { title: '请求数', dataIndex: 'request_count', width: 100, render: (_, r) => r.request_count.toLocaleString() },
+    { title: '输入 Token', dataIndex: 'input_tokens', width: 120, render: (_, r) => r.input_tokens.toLocaleString() },
+    { title: '输出 Token', dataIndex: 'output_tokens', width: 120, render: (_, r) => r.output_tokens.toLocaleString() },
+    {
+      title: '平均延迟',
+      dataIndex: 'avg_latency_ms',
+      width: 100,
+      render: (_, r) => r.avg_latency_ms ? `${Math.round(r.avg_latency_ms)}ms` : '-',
+    },
+    {
+      title: '成功率',
+      dataIndex: 'success_rate',
+      width: 100,
+      render: (_, r) => `${(r.success_rate * 100).toFixed(1)}%`,
+    },
+  ]
+
+  return (
+    <div>
+      <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', marginBottom: 24 }}>
+        <Title level={4} style={{ margin: 0 }}>用量统计</Title>
+        <Select
+          value={days}
+          onChange={setDays}
+          options={[
+            { value: 7, label: '最近 7 天' },
+            { value: 30, label: '最近 30 天' },
+            { value: 90, label: '最近 90 天' },
+          ]}
+          style={{ width: 140 }}
+        />
+      </div>
+
+      <Row gutter={[16, 16]} style={{ marginBottom: 24 }}>
+        <Col span={12}>
+          <Card>
+            <Statistic title="总请求数" value={totalRequests} prefix={<ThunderboltOutlined />} />
+          </Card>
+        </Col>
+        <Col span={12}>
+          <Card>
+            <Statistic title="总 Token 数" value={totalTokens} prefix={<ColumnWidthOutlined />} />
+          </Card>
+        </Col>
+      </Row>
+
+      <Card title="每日统计" style={{ marginBottom: 24 }} size="small">
+        <ProTable<DailyUsageStat>
+          columns={dailyColumns}
+          dataSource={dailyData ?? []}
+          loading={dailyLoading}
+          rowKey="day"
+          search={false}
+          toolBarRender={false}
+          pagination={false}
+          size="small"
+        />
+      </Card>
+
+      <Card title="按模型统计" size="small">
+        <ProTable<ModelUsageStat>
+          columns={modelColumns}
+          dataSource={modelData ?? []}
+          loading={modelLoading}
+          rowKey="model_id"
+          search={false}
+          toolBarRender={false}
+          pagination={false}
+          size="small"
+        />
+      </Card>
+    </div>
+  )
+}