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/task/auto_purchase.go

@@ -528,8 +528,8 @@ func (h *AutoPurchaseHandler) activateMainPackage(
 	}
 
 	if status == constants.PackageUsageStatusActive {
-		usage.ActivatedAt = activatedAt
-		usage.ExpiresAt = expiresAt
+		usage.ActivatedAt = &activatedAt
+		usage.ExpiresAt = &expiresAt
 		usage.NextResetAt = nextResetAt
 	}
 
@@ -588,7 +588,7 @@ func (h *AutoPurchaseHandler) activateAddonPackage(
 		Status:         constants.PackageUsageStatusActive,
 		Priority:       priority,
 		MasterUsageID:  &mainPackage.ID,
-		ActivatedAt:    now,
+		ActivatedAt:    &now,
 		ExpiresAt:      expiresAt,
 		DataResetCycle: pkg.DataResetCycle,
 		Generation:     order.Generation,