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/models/system_setting.py

@@ -20,7 +20,7 @@ class SystemSetting(Base):
     copyright_text: Mapped[str] = mapped_column(String(255), default="")
     theme_skin: Mapped[str] = mapped_column(String(64), default="sky")
 
-    admin_account: Mapped[str] = mapped_column(String(120), default="superadmin")
+    admin_account: Mapped[str] = mapped_column(String(120), default="admin")
     admin_email: Mapped[str] = mapped_column(String(255), default="")
     session_timeout: Mapped[int] = mapped_column(Integer, default=30)
     conversation_retention_days: Mapped[int] = mapped_column(Integer, default=3)