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/views/admin/Users.vue

@@ -250,7 +250,7 @@ import { parseApiError } from '@/utils/errorParser'
 import {
   entitlementsWillReplaceExisting,
   isPlanEntitlementReplacementCandidate,
-  usagePolicyEntitlementLabels,
+  billingEntitlementLabels,
 } from '@/utils/billingEntitlements'
 import { formatTokens, formatRateLimitInheritable, formatRateLimitSimple, isRateLimitInherited, isRateLimitUnlimited } from '@/utils/format'
 import { log } from '@/utils/logger'
@@ -563,21 +563,7 @@ function formatPlanDuration(plan: BillingPlan): string {
 }
 
 function entitlementLabels(items: BillingEntitlement[] | undefined): string[] {
-  return (items || []).flatMap((item) => {
-    if (item.type === 'wallet_credit') {
-      return `${legacyT('附赠余额')} $${Number(item.amount_usd || 0).toFixed(2)}`
-    }
-    if (item.type === 'daily_quota') {
-      return `${legacyT('每日额度')} $${Number(item.daily_quota_usd || 0).toFixed(2)}`
-    }
-    if (item.type === 'membership_group') {
-      return legacyT('会员权益')
-    }
-    if (item.type === 'usage_policy') {
-      return usagePolicyEntitlementLabels(item)
-    }
-    return []
-  })
+  return billingEntitlementLabels(items, legacyT)
 }
 
 function hasPackageEntitlement(items: BillingEntitlement[] | undefined): boolean {