Add unit tests for config

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

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

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

Start by locating the configuration handling and existing config unit tests; the issue names no files or test path. Confirm how validation notifications should behave, then verify that enable_notifications defaults to false and that the intended notification behavior is covered by tests.

索引モデルが issue の本文から書いたものです。