fix(auth): 登录目录就绪幂等化与并发控制

- employee/settings/user_session_metrics 的 ensure_*_ready 改为按 bind 缓存 + 锁，
  避免每次登录重复建表与并发场景下的竞态
- auth 登录链路先查员工再降级触发目录就绪，并吞掉查询期 SQLAlchemy 异常
- 默认管理员账号由 superadmin 迁移为 admin，兼容历史账号回填
- 补充登录降级与设置持久化相关测试

server/src/app/services/auth.py

@@ -5,6 +5,7 @@ from datetime import UTC, datetime, timedelta
 from typing import Any
 
 from sqlalchemy import func, or_, select
+from sqlalchemy.exc import SQLAlchemyError
 from sqlalchemy.orm import Session, selectinload
 
 from app.core.config import get_settings
@@ -127,8 +128,15 @@ class AuthService:
         if not self.settings.setup_completed:
             return None
 
-        EmployeeService(self.db).ensure_directory_ready()
-        employee = self._find_employee_by_email(identifier)
+        try:
+            employee = self._find_employee_by_email(identifier)
+        except SQLAlchemyError:
+            self.db.rollback()
+            employee = None
+
+        if employee is None:
+            EmployeeService(self.db).ensure_directory_ready()
+            employee = self._find_employee_by_email(identifier)
 
         if employee is None or not employee.password_hash:
             return None