Model层：新增金额分字段定义

- 所有 Model 添加对应的 *Cent int64 字段
- 旧的 float64 字段保留但不输出到 JSON (json:"-")
- 涉及文件：user.go, listing.go, order.go, order_checkout.go, wallet.go, withdrawal.go
- 目标：为后续业务逻辑切换到分存储做准备

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/model/withdrawal.go

@@ -6,9 +6,12 @@ type WithdrawalRequest struct {
 	ID               uint64     `gorm:"primaryKey" json:"id"`
 	WithdrawNo       string     `gorm:"size:64;not null;uniqueIndex" json:"withdraw_no"`
 	UserID           uint64     `gorm:"not null;index" json:"user_id"`
-	Amount           float64    `gorm:"type:decimal(12,2);not null" json:"amount"`
-	Fee              float64    `gorm:"type:decimal(12,2);not null;default:0.00" json:"fee"`
-	ActualAmount     float64    `gorm:"type:decimal(12,2);not null" json:"actual_amount"`
+	Amount           float64    `gorm:"type:decimal(12,2);not null" json:"-"`
+	AmountCent       int64      `gorm:"not null;default:0" json:"-"`
+	Fee              float64    `gorm:"type:decimal(12,2);not null;default:0.00" json:"-"`
+	FeeCent          int64      `gorm:"not null;default:0" json:"-"`
+	ActualAmount     float64    `gorm:"type:decimal(12,2);not null" json:"-"`
+	ActualAmountCent int64      `gorm:"not null;default:0" json:"-"`
 	PaymentAccountID *uint64    `json:"payment_account_id"`
 	AccountType      string     `gorm:"size:32;not null" json:"account_type"`
 	AccountName      string     `gorm:"size:128;not null" json:"account_name"`