refactor: 迁移收尾（管理员2FA）+ 抽取公共工具层，消除 router 间重复

补齐最后缺口：
- POST /api/v1/auth/admin_login/email、/auth/admin_login/verify（管理员 2FA Step2/3），
  复用 auth_routes 的票据表与尝试上限（跨栈共享），签发会话 Cookie

重构（消除约 200 行重复，11 个 router 受益）：
- web/request_utils.py：request_data / safe_int / extract_id / cookies / dedup_ids
  原在各 router 各复制一份，现集中
- web/serializers.py：format_credits / user_public / record_req_log
- 各 router 改为导入别名，行为逐字不变

覆盖率：Flask 90 条业务路由全部有 FastAPI 等价端点（含 5 个向后兼容别名）。
验证：pytest 69/69；ruff All checks passed；pyright 0 errors；奇偶 36/36；冒烟 10/10。

web/routers/admin.py

@@ -16,7 +16,7 @@ _get_cookie_info 仍为同步文件操作，经 asyncio.to_thread 执行。
 """
 import asyncio
 import logging
-from typing import Annotated, Any
+from typing import Annotated
 
 from fastapi import APIRouter, Depends, Request
 
@@ -24,6 +24,8 @@ from core.state import GLOBAL_MUSIC_U_CACHE, REQ_LOG, VIP_CACHE
 from web import ratelimit
 from web.deps import SiteUser, current_user
 from web.errors import AppError
+from web.request_utils import request_data as _request_data
+from web.request_utils import safe_int as _safe_int
 from web.response import error, success
 
 router = APIRouter()
@@ -41,32 +43,8 @@ async def _require_admin(user: Annotated[SiteUser | None, Depends(current_user)]
 AdminUser = Annotated[SiteUser, Depends(_require_admin)]
 
 
-async def _request_data(request: Request) -> dict[str, Any]:
-    data: dict[str, Any] = {}
-    if request.method == 'GET':
-        data.update(dict(request.query_params))
-        return data
-    try:
-        body = await request.json()
-        if isinstance(body, dict):
-            data.update(body)
-    except Exception:
-        pass
-    if not data:
-        try:
-            form = await request.form()
-            data.update(dict(form.items()))
-        except Exception:
-            pass
-    return data
 
 
-def _safe_int(value: Any, default: int) -> int:
-    try:
-        return int(value)
-    except (TypeError, ValueError):
-        return default
-
 
 def _admin_client():
     from qr_login import QRLoginClient