perf: 优化dashboard API，41次查询12次

- 合并30次循环查询为1次（onlineTrend）
- 使用 GROUP BY DATE() 一次获取所有日期数据
- 在内存中填充缺失日期，避免 N+1 问题
- API 响应时间预计从 1-2s 降至 100-200ms

backend/internal/router/admin/dashboard.go

@@ -1,4 +1,4 @@
-package admin
+package admin
 
 import (
 	"time"
@@ -111,23 +111,47 @@ func handleGetDashboard(c *gin.Context) {
 	}
 
 	onlineTrend := make([]gin.H, 0, 30)
-	for i := 29; i >= 0; i-- {
-		date := time.Now().AddDate(0, 0, -i)
-		dateStart := time.Date(date.Year(), date.Month(), date.Day(), 0, 0, 0, 0, date.Location())
-		dateEnd := dateStart.Add(24 * time.Hour)
+	if len(appIDs) > 0 {
+		type DailyCount struct {
+			Date  string
+			Count int64
+		}
+		var dailyCounts []DailyCount
 
-		var count int64
-		if len(appIDs) > 0 {
-			database.DB.Model(&model.DeviceSession{}).
-				Joins("JOIN user_devices ON device_sessions.device_id = user_devices.id").
-				Where("user_devices.application_id IN ? AND device_sessions.last_heartbeat >= ? AND device_sessions.last_heartbeat < ?", appIDs, dateStart, dateEnd).
-				Count(&count)
+		dateStart := time.Now().AddDate(0, 0, -29)
+		dateStart = time.Date(dateStart.Year(), dateStart.Month(), dateStart.Day(), 0, 0, 0, 0, dateStart.Location())
+
+		database.DB.Model(&model.DeviceSession{}).
+			Select("DATE(last_heartbeat) as date, COUNT(*) as count").
+			Joins("JOIN user_devices ON device_sessions.device_id = user_devices.id").
+			Where("user_devices.application_id IN ? AND last_heartbeat >= ?", appIDs, dateStart).
+			Group("DATE(last_heartbeat)").
+			Order("date ASC").
+			Find(&dailyCounts)
+
+		countMap := make(map[string]int64)
+		for _, dc := range dailyCounts {
+			countMap[dc.Date] = dc.Count
 		}
 
-		onlineTrend = append(onlineTrend, gin.H{
-			"date":  date.Format("01-02"),
-			"value": count,
-		})
+		for i := 29; i >= 0; i-- {
+			date := time.Now().AddDate(0, 0, -i)
+			dateStr := date.Format("2006-01-02")
+			count := countMap[dateStr]
+
+			onlineTrend = append(onlineTrend, gin.H{
+				"date":  date.Format("01-02"),
+				"value": count,
+			})
+		}
+	} else {
+		for i := 29; i >= 0; i-- {
+			date := time.Now().AddDate(0, 0, -i)
+			onlineTrend = append(onlineTrend, gin.H{
+				"date":  date.Format("01-02"),
+				"value": 0,
+			})
+		}
 	}
 
 	recentTickets := make([]gin.H, 0, 5)