perf: 全栈查询优化、前端缓存去重与页面可见性优化

后端:
- SQL count 查询统一改用 func.count() 子查询替代 query.count()
- Dashboard/Audit 等页面多次独立查询合并为单次聚合查询
- Provider summary 列表改为批量查询消除 N+1 问题
- DailyStats 逐天循环查询改为 CASE 分桶单次查询
- 使用 load_only() 减少不必要的列加载
- cache_decorator 支持嵌套属性路径解析(dotted vary_by)
- 多个管理/公共端点新增 @cache_result 缓存装饰

前端:
- cache.ts 新增 in-flight 请求复用、dedupedRequest、buildCacheKey
- 大量 API 调用添加前端缓存或去重
- 多个页面定时器在标签页隐藏时暂停、可见时恢复
- Auth 检查从 setInterval 改为 storage + visibilitychange 事件驱动
- 请求竞态防护(requestId 模式)

数据库:
- Usage 表新增 idx_usage_status_user_created 复合索引

frontend/src/views/shared/Dashboard.vue

@@ -987,6 +987,10 @@ const dailyTimeRange = ref<DateRangeParams>(getDateRangeFromPeriod('last7days'))
 // 统计周期
 const loadingDaily = ref(false)
 const loading = ref(false)
+let dailyStatsRequestId = 0
+let dailyStatsLoadPromise: Promise<void> | null = null
+let hasPendingDailyStatsLoad = false
+let dailyStatsDebounceTimer: ReturnType<typeof setTimeout> | null = null
 
 
 // 公告
@@ -1279,7 +1283,8 @@ onMounted(async () => {
   }
   await Promise.all([
     loadDashboardData(),
-    loadAnnouncements()
+    loadAnnouncements(),
+    loadDailyStats()
   ])
   await nextTick()
   setupTimelineResizeObserver()
@@ -1298,6 +1303,13 @@ onBeforeUnmount(() => {
   statsPanelObserver = null
   announcementsTimelineObserver?.disconnect()
   announcementsTimelineObserver = null
+  if (dailyStatsDebounceTimer) {
+    clearTimeout(dailyStatsDebounceTimer)
+    dailyStatsDebounceTimer = null
+  }
+  hasPendingDailyStatsLoad = false
+  dailyStatsLoadPromise = null
+  dailyStatsRequestId += 1
 })
 
 async function loadDashboardData() {
@@ -1326,22 +1338,48 @@ async function loadDashboardData() {
 }
 
 async function loadDailyStats() {
-  loadingDaily.value = true
-  try {
-    const response = await dashboardApi.getDailyStats(dailyTimeRange.value)
-    dailyStats.value = response.daily_stats
-    providerSummary.value = response.provider_summary || []
-  } catch {
-    dailyStats.value = []
-    providerSummary.value = []
-  } finally {
-    loadingDaily.value = false
+  if (dailyStatsLoadPromise) {
+    hasPendingDailyStatsLoad = true
+    return dailyStatsLoadPromise
   }
+  const requestId = ++dailyStatsRequestId
+  loadingDaily.value = true
+  dailyStatsLoadPromise = (async () => {
+    try {
+      const response = await dashboardApi.getDailyStats(dailyTimeRange.value)
+      if (requestId !== dailyStatsRequestId) return
+      dailyStats.value = response.daily_stats
+      providerSummary.value = response.provider_summary || []
+    } catch {
+      if (requestId !== dailyStatsRequestId) return
+      dailyStats.value = []
+      providerSummary.value = []
+    } finally {
+      if (requestId === dailyStatsRequestId) {
+        loadingDaily.value = false
+      }
+    }
+  })().finally(() => {
+    dailyStatsLoadPromise = null
+    if (hasPendingDailyStatsLoad) {
+      hasPendingDailyStatsLoad = false
+      void loadDailyStats()
+    }
+  })
+  return dailyStatsLoadPromise
 }
 
-watch(dailyTimeRange, async () => {
-  await loadDailyStats()
-}, { deep: true })
+function scheduleDailyStatsLoad() {
+  if (dailyStatsDebounceTimer) {
+    clearTimeout(dailyStatsDebounceTimer)
+  }
+  dailyStatsDebounceTimer = setTimeout(() => {
+    dailyStatsDebounceTimer = null
+    void loadDailyStats()
+  }, 120)
+}
+
+watch(dailyTimeRange, scheduleDailyStatsLoad, { deep: true })
 
 function formatDate(dateString: string): string {
   const date = new Date(dateString)