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/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
+)