chore: 清理两处未使用代码

- app/db/base.py: 删 MappedAsDataclass 的 import (没人继承用)
- app/integrations/wxpay.py: 删 code_to_openid 函数 (code_to_userinfo 已覆盖能力, 18 行死代码)

app/db/base.py

@@ -4,7 +4,7 @@
 """
 from __future__ import annotations
 
-from sqlalchemy.orm import DeclarativeBase, MappedAsDataclass
+from sqlalchemy.orm import DeclarativeBase
 
 
 class Base(DeclarativeBase):

app/integrations/wxpay.py

@@ -162,24 +162,6 @@ def cancel_transfer(out_bill_no: str) -> dict:
     return {"status_code": resp.status_code, "data": resp.json()}
 
 
-def code_to_openid(code: str) -> str:
-    """用微信授权 code 换 openid(开放平台移动应用 sns/oauth2)。失败抛 ValueError。"""
-    resp = httpx.get(
-        "https://api.weixin.qq.com/sns/oauth2/access_token",
-        params={
-            "appid": settings.WECHAT_APP_ID,
-            "secret": settings.WECHAT_APP_SECRET,
-            "code": code,
-            "grant_type": "authorization_code",
-        },
-        timeout=settings.WXPAY_REQUEST_TIMEOUT_SEC,
-    )
-    data = resp.json()
-    if "openid" not in data:
-        raise ValueError(f"微信授权失败: {data.get('errmsg', data)}")
-    return data["openid"]
-
-
 def code_to_userinfo(code: str) -> dict:
     """code 换 access_token+openid,再拉 sns/userinfo 取昵称头像。
     返回 {openid, nickname, avatar_url, raw}。失败抛 ValueError。