docs: 提现允许在途时继续提交申请 设计spec (#173)

## 摘要
取消「同一用户同时仅一笔在途提现」限制:已有 reviewing/pending 提现单时可继续发起新申请。

- 删应用层在途单检查(WithdrawTooFrequentError)+ 删 DB 分区唯一索引 ux_withdraw_order_user_active(含迁移)
- 清理失效死代码;IntegrityError 兜底瘦身为仅处理 out_bill_no 幂等
- 既有约束不变:建单先扣款(防超提)、coin_cash 每日档位次数、out_bill_no 幂等、解绑退款、admin 审核/对账均按单号维度

## 测试
- 新增:多笔在途并存放行(coin_cash & invite_cash)、第二笔仅受余额约束(409 现金余额不足)
- 迁移 upgrade→downgrade→upgrade 回环验证
- 提现域全绿(test_withdraw / test_invite_cash_withdraw / test_withdraw_ledger_check)

## 注意
- 客户端:每次提交需生成新的 out_bill_no;未开免确认时多笔 pending 各返回一个微信确认页,App 需能处理多笔待确认
- 无并发硬上限(产品拍板):coin_cash 由每日档位次数天然封顶,invite_cash 仅受余额约束

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Co-authored-by: guke <guke@autohome.com.cn>
Reviewed-on: #173

app/api/v1/wallet.py

@@ -222,11 +222,6 @@ def withdraw(req: WithdrawRequest, user: CurrentUser, db: DbSession) -> Withdraw
         ) from e
     except crud_wallet.WechatNotBoundError as e:
         raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="请先绑定微信") from e
-    except crud_wallet.WithdrawTooFrequentError as e:
-        raise HTTPException(
-            status_code=status.HTTP_409_CONFLICT,
-            detail="已有提现申请正在审核或打款中,请处理完成后再申请",
-        ) from e
     except crud_wallet.WithdrawTierUnavailableError as e:
         # 福利页档位闸(7-9):次数满/已选其他额度。正常客户端已按 tiers 预拦,此处兜底防绕过。
         raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail="今日额度已达上限") from e