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/polling/initializer.go

@@ -262,9 +262,12 @@ func (p *PollingInitializer) initBatch(ctx context.Context, cards []*model.IotCa
 
 		cacheKey := constants.RedisPollingCardInfoKey(card.ID)
 		cacheData := map[string]interface{}{
-			"id": card.ID, "iccid": card.ICCID,
-			"card_category": card.CardCategory, "real_name_status": card.RealNameStatus,
-			"network_status": card.NetworkStatus, "carrier_id": card.CarrierID,
+			"id":                      card.ID,
+			"iccid":                   card.ICCID,
+			"card_category":           card.CardCategory,
+			"real_name_status":        card.RealNameStatus,
+			"network_status":          card.NetworkStatus,
+			"carrier_id":              card.CarrierID,
 			"current_month_usage_mb":  card.CurrentMonthUsageMB,
 			"last_gateway_reading_mb": card.LastGatewayReadingMB,
 			"data_usage_mb":           card.DataUsageMB,