fix(billing): 异步任务退款时同步减少 used_quota (#6795)

* fix(billing): 异步任务退款时同步减少 used_quota
退款时仅恢复了 quota（剩余额度），但未同步减少 used_quota（已用额度），
导致"总额度"（quota + used_quota）随退款次数持续虚增，超出用户实际充值金额。

修复三处退款路径：
- RefundTaskQuota：任务失败完整退款
- RecalculateTaskQuota：差额结算退款分支
- controller/midjourney.go：Midjourney 任务失败退款

新增 model.UpdateUserUsedQuota 公开函数，仅调整 used_quota 不影响 request_count。

* fix(billing): 任务退款时同步扣减渠道 used_quota

* fix(billing): complete async task refund accounting

* style(model): group internal Midjourney fields

---------

Co-authored-by: CaIon <i@caion.me>

model/midjourney.go

@@ -23,6 +23,9 @@ type Midjourney struct {
 	Quota       int    `json:"quota"`
 	Buttons     string `json:"buttons"`
 	Properties  string `json:"properties"`
+
+	TokenId          int `json:"-" gorm:"default:0"`
+	BillingChannelId int `json:"-" gorm:"default:0"`
 }
 
 // TaskQueryParams 用于包含所有搜索条件的结构体，可以根据需求添加更多字段
@@ -170,6 +173,19 @@ func (midjourney *Midjourney) Update() error {
 	return err
 }
 
+func (midjourney *Midjourney) UpdateBillingState() error {
+	return DB.Model(midjourney).
+		Select("quota", "token_id", "billing_channel_id").
+		Updates(midjourney).Error
+}
+
+func (midjourney *Midjourney) GetBillingChannelId() int {
+	if midjourney.BillingChannelId > 0 {
+		return midjourney.BillingChannelId
+	}
+	return midjourney.ChannelId
+}
+
 // UpdateWithStatus performs a conditional UPDATE guarded by fromStatus (CAS).
 // Returns (true, nil) if this caller won the update, (false, nil) if
 // another process already moved the task out of fromStatus.