FDP v2 index DELETE index/entries/{uuid} fails

DELETE requests to `index/entries/{uuid}` fail with status 500

FDP logs show:

```none
...
org.postgresql.util.PSQLException: ERROR: update or delete on table "index_entry" violates foreign key constraint "fk__index_event_related_to" on table "index_event"
  Detail: Key (uuid)=(977d24d4-69b4-4e7b-b4a7-d8d1c0c7451f) is still referenced from table "index_event".
...
```