fix: 修正套餐激活和时间字段nullable问题

核心变更：
1. Model层时间字段改为*time.Time并设为nullable
   - PackageUsage.ActivatedAt/ExpiresAt
   - PersonalCustomerDevice/ICCID/Phone.LastUsedAt/VerifiedAt

2. 数据库迁移：
   - activated_at/expires_at列移除NOT NULL约束
   - 清洗零值记录(status=0且activated_at<'2000-01-01')

3. 新增ActivateSpecificPackage方法：精准激活指定套餐，
   修复HandlePackageQueueActivation从"查找过期包"改为直接激活payload指定套餐

4. 新增孤儿套餐恢复扫描：Worker启动或每次套餐检查时，
   自动发现并恢复无status=1主套餐的孤儿载体

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/handler/app/client_asset.go

@@ -363,8 +363,8 @@ func (h *ClientAssetHandler) GetPackageHistory(c *fiber.Ctx) error {
 			VirtualUsedMB:   float64(usage.DataUsageMB) / ratio,
 			VirtualRemainMB: float64(usage.DataLimitMB-usage.DataUsageMB) / ratio,
 			VirtualRatio:    ratio,
-			ActivatedAt:     nonZeroTimePtr(usage.ActivatedAt),
-			ExpiresAt:       nonZeroTimePtr(usage.ExpiresAt),
+			ActivatedAt:     usage.ActivatedAt,
+			ExpiresAt:       usage.ExpiresAt,
 			MasterUsageID:   usage.MasterUsageID,
 			Priority:        usage.Priority,
 			CreatedAt:       usage.CreatedAt,