Default "Is audio?" to true in admin form

## Description

`mediaIsAudio` defaults to `true` in the entity (`src/Entity/PointOfInterest.php:98`), but the admin form should explicitly show this pre-checked for new POIs.

## Implementation

Update the `BooleanField` in `src/Controller/Admin/PointOfInterestCrudController.php` to ensure the default value is reflected in the admin form when creating new Points of Interest.

Part of #35