Restrict deletion of features other features depend on

Add dependency validation to `DELETE /api/v1/projects/{project_pk}/features/{id}/`.

### Acceptance criteria

- Deleting (not archiving) a feature is rejected with a 400 while any `SegmentFlagReference` row points at it.
- Removing the last reference allows deletion to proceed.

Start at the DELETE /api/v1/projects/{project_pk}/features/{id}/ endpoint and trace how feature deletion differs from archiving. Inspect the SegmentFlagReference relationship and add coverage for rejection while references exist and successful deletion after the last reference is removed; done means the stated 400 behavior and deletion behavior are verified.

Written by the indexing model from the issue text.