Interface still reflects cached assignments even after assigned Network object is deleted

When a `Network` address is assigned to an `Interface` and the underlying `Network` object is deleted from the database, the `Interface._addresses_cache` and `Interface._networks_cache` are no updated. This is an inconsistency that needs to be addressed.

Possible solutions:
- A `Network` object cannot be deleted as long as it has an existing `Assignment` relationship to an `Interface`
- Deleting a `Network` with an `Assignment` in place also triggers an update to the `Interface` to refresh its cached relationships.