💡(backend) warm about the token nature of Yprovider microservice

Note to the future myself, using a raw token format is
not common. It should be refactor

src/backend/core/services/converter_services.py

@@ -31,6 +31,7 @@ class YdocConverter:
     @property
     def auth_header(self):
         """Build microservice authentication header."""
+        # Note: Yprovider microservice accepts only raw token, which is not recommended
         return settings.Y_PROVIDER_API_KEY
 
     def convert_markdown(self, text):