Consider deprecating old middleware API

**Feature request**
It would be great to have notifications for models.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_notifications` with default `false`.

The issue names no files, tests, or entry points. Start by locating the configuration handling and the model-related notification flow, then determine the expected notification behavior and add coverage showing that the new option defaults to false and works when enabled.

Written by the indexing model from the issue text.