fix(workload): get_people_workload 内嵌 _to_bj 改用 to_beijing，修复 BEIJING_TZ 未定义

- 移除 import 后内嵌 _to_bj 仍引用 BEIJING_TZ 导致 NameError

backend/app/services/dashboard_service.py

@@ -710,14 +710,7 @@ async def get_people_workload(db: AsyncSession) -> list[PersonWorkload]:
     rows = result.all()
 
     def _to_bj(dt):
-        if not dt:
-            return None
-        if dt.tzinfo is None:
-            from datetime import timezone as dt_timezone
-            dt = dt.replace(tzinfo=dt_timezone.utc).astimezone(BEIJING_TZ)
-        else:
-            dt = dt.astimezone(BEIJING_TZ)
-        return dt
+        return to_beijing(dt)
 
     now = get_beijing_time()