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/package/reset_service.go

@@ -138,9 +138,9 @@ func (s *ResetService) resetMonthlyUsageWithDB(ctx context.Context, db *gorm.DB)
 				continue
 			}
 
-			activatedAt := usage.ActivatedAt
-			if activatedAt.IsZero() {
-				activatedAt = now
+			activatedAt := now
+			if usage.ActivatedAt != nil && !usage.ActivatedAt.IsZero() {
+				activatedAt = *usage.ActivatedAt
 			}
 			nextResetAt := CalculateNextResetTime(constants.PackageDataResetMonthly, pkg.CalendarType, now, activatedAt)
 			if nextResetAt == nil {