feat: implement withdrawal improvements P01-P05

- P04: fix Withdrawal.TableName() to return 'withdrawals'
- P03: add WithdrawalStatus* and WithdrawalMethod* constants; replace magic numbers; add CommissionTypeWithdrawCancel=338
- P01: add POST /v1/public/user/withdrawal_cancel to cancel pending withdrawals with ownership check, row lock, commission refund, and audit log
- P02: add method/account/qr_code_url fields to Withdrawal model and API; add input validation (qr_code_url required for Alipay/Wechat, account required for bank)
- P05: add method filter to admin GET /v1/admin/user/withdrawal/list

Co-authored-by: multica-agent <github@multica.ai>

internal/model/user/user.go

@@ -167,12 +167,15 @@ type Withdrawal struct {
 	UserId    int64     `gorm:"index:idx_user_id;not null;comment:User ID"`
 	Amount    int64     `gorm:"not null;comment:Withdrawal Amount"`
 	Content   string    `gorm:"type:text;comment:Withdrawal Content"`
-	Status    uint8     `gorm:"type:tinyint(1);default:0;comment:Withdrawal Status: 0: Pending 1: Approved 2: Rejected"`
+	Status    uint8     `gorm:"type:tinyint(1);default:0;comment:Withdrawal Status: 0: Pending 1: Approved 2: Rejected 3: Cancelled"`
 	Reason    string    `gorm:"type:varchar(500);default:'';comment:Rejection Reason"`
+	Method    uint8     `gorm:"type:tinyint(1);default:0;comment:收款方式 0:其他 1:支付宝 2:微信 3:银行卡"`
+	Account   string    `gorm:"type:varchar(255);default:'';comment:收款账号"`
+	QrCodeUrl string    `gorm:"type:varchar(500);default:'';comment:收款码图片URL"`
 	CreatedAt time.Time `gorm:"<-:create;comment:Creation Time"`
 	UpdatedAt time.Time `gorm:"comment:Update Time"`
 }
 
 func (*Withdrawal) TableName() string {
-	return "user_withdrawal"
+	return "withdrawals"
 }

internal/model/user/withdrawal_const.go

@@ -0,0 +1,15 @@
+package user
+
+const (
+	WithdrawalStatusPending   uint8 = 0 // 待审核
+	WithdrawalStatusApproved  uint8 = 1 // 已通过
+	WithdrawalStatusRejected  uint8 = 2 // 已拒绝
+	WithdrawalStatusCancelled uint8 = 3 // 已取消（用户自行撤回）
+)
+
+const (
+	WithdrawalMethodOther  uint8 = 0 // 其他
+	WithdrawalMethodAlipay uint8 = 1 // 支付宝
+	WithdrawalMethodWechat uint8 = 2 // 微信
+	WithdrawalMethodBank   uint8 = 3 // 银行卡
+)