refac: legacy support

backend/open_webui/migrations/versions/6a39f3d8e55c_add_knowledge_table.py

@@ -65,6 +65,7 @@ def upgrade():
                 user_id=doc.user_id,
                 description=doc.name,
                 meta={
+                    "legacy": True,
                     "document": True,
                     "tags": json.loads(doc.content or "{}").get("tags", []),
                 },