feat: revamp analytics dashboards and harden database migrations

Add dashboard and overview analytics, health monitoring, provider expense tracking, and announcement updates across the gateway and frontend.

Keep schema migrations free of historical backfills while preserving automatic backfill execution. Bound migration deadlines, run schema preparation before Compose replacement, and anonymize deleted dashboard users.

Include the current documentation cleanup and regression coverage.

frontend/src/features/overview/components/OverviewTrend.vue

@@ -0,0 +1,49 @@
+<template>
+  <section class="min-w-0 space-y-3">
+    <div class="flex items-center justify-between gap-3">
+      <h2 class="text-sm font-semibold">
+        {{ title || t('使用趋势', 'Usage trend') }}
+      </h2><select
+        v-model="metric"
+        :aria-label="t('趋势指标', 'Trend metric')"
+        class="h-8 rounded-md border bg-background px-2 text-xs"
+      >
+        <option value="requests">
+          {{ t('请求数', 'Requests') }}
+        </option><option value="tokens">
+          Tokens
+        </option><option value="billable">
+          {{ t('计费消耗', 'Billable consumption') }}
+        </option>
+      </select>
+    </div>
+    <div
+      v-if="!points.length"
+      class="flex h-64 items-center justify-center text-sm text-muted-foreground"
+    >
+      {{ t('此时间范围暂无数据', 'No data in this range') }}
+    </div>
+    <div
+      v-else
+      class="h-64 min-w-0"
+    >
+      <LineChart :data="chart" />
+    </div>
+  </section>
+</template>
+
+<script setup lang="ts">
+import { computed, ref } from 'vue'
+import type { OverviewSeriesPoint } from '@/api/overview'
+import LineChart from '@/components/charts/LineChart.vue'
+import { timestamp } from '../format'
+import { useOverviewI18n } from '../i18n'
+const props = defineProps<{ points: OverviewSeriesPoint[]; timezone: string; title?: string }>()
+const { t } = useOverviewI18n()
+const metric = ref('requests')
+const chart = computed(() => ({ labels: props.points.map(p => timestamp(p.bucket_start, props.timezone)), datasets: [{
+  label: metric.value === 'tokens' ? 'Tokens' : metric.value === 'billable' ? t('计费消耗 (USD)', 'Billable consumption (USD)') : t('请求数', 'Requests'),
+  data: props.points.map(p => metric.value === 'tokens' ? p.total_tokens : metric.value === 'billable' ? (p.billable_amount?.value == null ? null : Number(p.billable_amount.value)) : p.request_count),
+  borderColor: '#0d9488', backgroundColor: '#0d9488', pointRadius: props.points.length > 60 ? 0 : 2, borderWidth: 2, tension: 0.15,
+}] }))
+</script>