修复编译错误：添加strings包导入和修复时间类型转换

- announcements.go: 添加strings包导入
- devices.go: 添加strings包导入
- cloud.go: 修复CreatedAt时间类型转换为字符串

backend/internal/router/admin/announcements.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"log"
 	"strconv"
+	"strings"
 
 	"verification-platform-backend/internal/database"
 	"verification-platform-backend/internal/model"