feat: 统计数据优化 - 支持细粒度时间范围和多维度分析

- 新增 StatsHourly/StatsDaily 预聚合表，支持任意时区的精确统计
- 实现 UTC datetime 范围查询策略，边界数据实时聚合
- 新增统计 API：用户/API Key 维度、成本分析、性能百分位、错误分类
- 新增前端页面：成本分析、性能分析、用户统计
- 新增 TimeRangePicker 组件和统计可视化组件
- 优化 Dashboard 和 Usage 页面支持时间范围筛选

Close #135

frontend/src/components/stats/LeaderboardTable.vue

@@ -0,0 +1,95 @@
+<template>
+  <TableCard :title="title">
+    <template #actions>
+      <Select
+        v-if="showMetricSelect"
+        v-model:open="metricSelectOpen"
+        :model-value="metric"
+        @update:model-value="emitMetric"
+      >
+        <SelectTrigger class="h-8 text-xs w-28">
+          <SelectValue placeholder="指标" />
+        </SelectTrigger>
+        <SelectContent>
+          <SelectItem value="requests">请求数</SelectItem>
+          <SelectItem value="tokens">Tokens</SelectItem>
+          <SelectItem value="cost">成本</SelectItem>
+        </SelectContent>
+      </Select>
+    </template>
+
+    <div v-if="loading" class="p-6">
+      <LoadingState />
+    </div>
+    <div v-else-if="items.length === 0" class="p-6">
+      <EmptyState title="暂无数据" description="当前时间范围内没有统计结果" />
+    </div>
+    <Table v-else>
+      <TableHeader>
+        <TableRow>
+          <TableHead class="w-16">排名</TableHead>
+          <TableHead>名称</TableHead>
+          <TableHead class="text-right">请求数</TableHead>
+          <TableHead class="text-right">Tokens</TableHead>
+          <TableHead class="text-right">成本</TableHead>
+        </TableRow>
+      </TableHeader>
+      <TableBody>
+        <TableRow v-for="item in items" :key="item.id">
+          <TableCell class="font-medium">{{ item.rank }}</TableCell>
+          <TableCell>{{ item.name }}</TableCell>
+          <TableCell class="text-right">{{ item.requests }}</TableCell>
+          <TableCell class="text-right">{{ formatTokens(item.tokens) }}</TableCell>
+          <TableCell class="text-right">{{ formatCurrency(item.cost) }}</TableCell>
+        </TableRow>
+      </TableBody>
+    </Table>
+  </TableCard>
+</template>
+
+<script setup lang="ts">
+import { computed, ref } from 'vue'
+import { EmptyState, LoadingState } from '@/components/common'
+import { TableCard } from '@/components/ui'
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+  Table,
+  TableBody,
+  TableCell,
+  TableHead,
+  TableHeader,
+  TableRow
+} from '@/components/ui'
+import { formatCurrency, formatTokens } from '@/utils/format'
+import type { LeaderboardItem } from '@/api/admin'
+
+interface Props {
+  title: string
+  items: LeaderboardItem[]
+  metric: 'requests' | 'tokens' | 'cost'
+  loading?: boolean
+  showMetricSelect?: boolean
+}
+
+const props = withDefaults(defineProps<Props>(), {
+  loading: false,
+  showMetricSelect: true
+})
+
+const emit = defineEmits<{
+  (e: 'update:metric', value: 'requests' | 'tokens' | 'cost'): void
+}>()
+
+const metric = computed(() => props.metric)
+const metricSelectOpen = ref(false)
+
+function emitMetric(value: string) {
+  if (value === 'requests' || value === 'tokens' || value === 'cost') {
+    emit('update:metric', value)
+  }
+}
+</script>