fix: 修复账号列表首次加载窗口费用显示 $0.00

lite 模式下从快照缓存读取窗口费用，非 lite 模式查询后写入缓存

backend/internal/handler/admin/account_window_cost_cache.go

@@ -0,0 +1,25 @@
+package admin
+
+import (
+	"strconv"
+	"strings"
+	"time"
+)
+
+var accountWindowCostCache = newSnapshotCache(30 * time.Second)
+
+func buildWindowCostCacheKey(accountIDs []int64) string {
+	if len(accountIDs) == 0 {
+		return "accounts_window_cost_empty"
+	}
+	var b strings.Builder
+	b.Grow(len(accountIDs) * 6)
+	_, _ = b.WriteString("accounts_window_cost:")
+	for i, id := range accountIDs {
+		if i > 0 {
+			_ = b.WriteByte(',')
+		}
+		_, _ = b.WriteString(strconv.FormatInt(id, 10))
+	}
+	return b.String()
+}