correct mismatched kb doc number (#826)

### What problem does this PR solve?

#620

### Type of change

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

api/db/services/document_service.py

@@ -265,3 +265,9 @@ class DocumentService(CommonService):
             except Exception as e:
                 stat_logger.error("fetch task exception:" + str(e))
 
+    @classmethod
+    @DB.connection_context()
+    def get_kb_doc_count(cls, kb_id):
+        return len(cls.model.select(cls.model.id).where(
+            cls.model.kb_id == kb_id).dicts())
+