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/service/asset/service.go

@@ -687,8 +687,8 @@ func (s *Service) GetPackages(ctx context.Context, assetType string, id uint, pa
 			VirtualRemainMB:   float64(u.DataLimitMB-u.DataUsageMB) / ratio,
 			VirtualRatio:      ratio,
 			EnableVirtualData: enableVirtualData,
-			ActivatedAt:       nonZeroTimePtr(u.ActivatedAt),
-			ExpiresAt:         nonZeroTimePtr(u.ExpiresAt),
+			ActivatedAt:       u.ActivatedAt,
+			ExpiresAt:         u.ExpiresAt,
 			MasterUsageID:     u.MasterUsageID,
 			Priority:          u.Priority,
 			CreatedAt:         u.CreatedAt,
@@ -764,8 +764,8 @@ func (s *Service) GetCurrentPackage(ctx context.Context, assetType string, id ui
 		VirtualRemainMB:   float64(usage.DataLimitMB-usage.DataUsageMB) / ratio,
 		VirtualRatio:      ratio,
 		EnableVirtualData: enableVirtualData,
-		ActivatedAt:       nonZeroTimePtr(usage.ActivatedAt),
-		ExpiresAt:         nonZeroTimePtr(usage.ExpiresAt),
+		ActivatedAt:       usage.ActivatedAt,
+		ExpiresAt:         usage.ExpiresAt,
 		MasterUsageID:     usage.MasterUsageID,
 		Priority:          usage.Priority,
 		CreatedAt:         usage.CreatedAt,