fix thumbnail issue (#2917)

### What problem does this PR solve?


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/db/services/document_service.py

     @classmethod
     @DB.connection_context()
     def get_thumbnails(cls, docids):
-        fields = [cls.model.id, cls.model.thumbnail]
+        fields = [cls.model.id, cls.model.kb_id, cls.model.thumbnail]
         return list(cls.model.select(
             *fields).where(cls.model.id.in_(docids)).dicts())