feat: add expired share cleanup functionality

services/share.py

@@ -90,6 +90,16 @@ class ShareService:
             raise HTTPException(status_code=404, detail="分享链接不存在")
         await share.delete()
 
+    @staticmethod
+    async def delete_expired_shares(user: UserAccount) -> int:
+        """
+        删除当前用户所有已过期的分享链接，返回删除数量。
+        条件：expires_at 非空 且 小于等于当前时间（UTC）。
+        """
+        now = datetime.now(timezone.utc)
+        deleted_count = await ShareLink.filter(user=user, expires_at__lte=now).delete()
+        return deleted_count
+
     @staticmethod
     async def get_shared_item_details(share: ShareLink, sub_path: str = ""):
         """
@@ -122,4 +132,4 @@ class ShareService:
             raise e
 
 
-share_service = ShareService()
+share_service = ShareService()