fix(duty): drop unused import in duty admin API

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/api/duty.py

@@ -1,7 +1,7 @@
 """值班排班 · 管理端 API"""
 from __future__ import annotations
 
-from datetime import date, datetime
+from datetime import datetime
 from typing import Optional
 
 from fastapi import APIRouter, Depends, Query