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_handler.go

@@ -288,8 +288,19 @@ func (h *AccountHandler) List(c *gin.Context) {
 		}
 	}
 
-	// 仅非 lite 模式获取窗口费用（PostgreSQL 聚合查询，高开销）
-	if !lite && len(windowCostAccountIDs) > 0 {
+	// 窗口费用获取：lite 模式从快照缓存读取，非 lite 模式执行 PostgreSQL 查询后写入缓存
+	if len(windowCostAccountIDs) > 0 {
+		if lite {
+			// lite 模式：尝试从快照缓存读取
+			cacheKey := buildWindowCostCacheKey(windowCostAccountIDs)
+			if cached, ok := accountWindowCostCache.Get(cacheKey); ok {
+				if costs, ok := cached.Payload.(map[int64]float64); ok {
+					windowCosts = costs
+				}
+			}
+			// 缓存未命中则 windowCosts 保持 nil（仅发生在服务刚启动时）
+		} else {
+			// 非 lite 模式：执行 PostgreSQL 聚合查询（高开销）
 			windowCosts = make(map[int64]float64)
 			var mu sync.Mutex
 			g, gctx := errgroup.WithContext(c.Request.Context())
@@ -314,6 +325,11 @@ func (h *AccountHandler) List(c *gin.Context) {
 				})
 			}
 			_ = g.Wait()
+
+			// 查询完毕后写入快照缓存，供 lite 模式使用
+			cacheKey := buildWindowCostCacheKey(windowCostAccountIDs)
+			accountWindowCostCache.Set(cacheKey, windowCosts)
+		}
 	}
 
 	// Build response with concurrency info

backend/internal/handler/admin/account_window_cost_cache.go

+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()
+}