feat(ia): polish project operation workspaces

backend/app/crud/overview.py

@@ -1,6 +1,6 @@
 """概览数据聚合CRUD操作"""
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Dict, List, Tuple
 
 from sqlalchemy import Date, cast, func, select, and_, or_
@@ -34,7 +34,7 @@ async def get_project_overview(db: AsyncSession, study_id: uuid.UUID) -> Dict:
     if not sites:
         return {
             "study_id": str(study_id),
-            "updated_at": datetime.now().isoformat(),
+            "updated_at": datetime.now(timezone.utc).isoformat(),
             "centers": [],
             "enrollment_by_month": []
         }
@@ -138,7 +138,7 @@ async def get_project_overview(db: AsyncSession, study_id: uuid.UUID) -> Dict:
     
     return {
         "study_id": str(study_id),
-        "updated_at": datetime.now().isoformat(),
+        "updated_at": datetime.now(timezone.utc).isoformat(),
         "centers": centers,
         "enrollment_by_month": monthly_enrollment
     }