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/model/package.go

@@ -69,8 +69,8 @@ type PackageUsage struct {
 	DataUsageMB               int64      `gorm:"column:data_usage_mb;type:bigint;default:0;comment:已使用流量(MB)" json:"data_usage_mb"`
 	RealDataUsageMB           int64      `gorm:"column:real_data_usage_mb;type:bigint;default:0;comment:真流量使用(MB)" json:"real_data_usage_mb"`
 	VirtualDataUsageMB        int64      `gorm:"column:virtual_data_usage_mb;type:bigint;default:0;comment:虚流量使用(MB)" json:"virtual_data_usage_mb"`
-	ActivatedAt               time.Time  `gorm:"column:activated_at;not null;comment:套餐生效时间" json:"activated_at"`
-	ExpiresAt                 time.Time  `gorm:"column:expires_at;not null;comment:套餐过期时间" json:"expires_at"`
+	ActivatedAt               *time.Time `gorm:"column:activated_at;comment:套餐生效时间" json:"activated_at"`
+	ExpiresAt                 *time.Time `gorm:"column:expires_at;comment:套餐过期时间" json:"expires_at"`
 	Status                    int        `gorm:"column:status;type:int;default:1;not null;comment:状态 0-待生效 1-生效中 2-已用完 3-已过期 4-已失效" json:"status"`
 	LastPackageCheckAt        *time.Time `gorm:"column:last_package_check_at;comment:最后一次套餐流量检查时间" json:"last_package_check_at"`
 	Priority                  int        `gorm:"column:priority;type:int;default:1;index:idx_package_usage_priority;comment:优先级(主套餐和加油包按此字段排队,数字越小优先级越高)" json:"priority"`