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/i18n/messages.ts

@@ -345,7 +345,7 @@ export const messages = {
     'nav.myReferral': '我的邀请',
     'nav.usageStats': '使用统计',
     'nav.operations': '运维总览',
-    'nav.userStats': '用户统计',
+    'nav.userStats': '用户分析',
     'nav.costAnalysis': '成本分析',
     'nav.performanceAnalysis': '性能分析',
     'nav.userManagement': '用户管理',
@@ -791,7 +791,7 @@ export const messages = {
     'nav.myReferral': 'Referrals',
     'nav.usageStats': 'Usage stats',
     'nav.operations': 'Operations',
-    'nav.userStats': 'User stats',
+    'nav.userStats': 'User analysis',
     'nav.costAnalysis': 'Cost analysis',
     'nav.performanceAnalysis': 'Performance analysis',
     'nav.userManagement': 'Users',
@@ -1121,6 +1121,9 @@ const legacyExactEnglishMessages: Record<string, string> = {
   '暂无连接事件记录': 'No connection events',
   '节点数据': 'Node data',
   '最近 24 小时': 'Last 24 hours',
+  '最近 1 小时': 'Last hour',
+  '已选时段': 'Selected range',
+  '时间范围': 'Time range',
   '加载节点数据...': 'Loading node data...',
   '加载节点数据失败': 'Failed to load node data',
   '加载事件失败': 'Failed to load events',