fix: hard delete EnterpriseGroup records instead of soft deleting them (ENT-12073) (#2660)

Soft-deleted groups remained in the database and collided with the
unique_together constraint on (name, enterprise_customer), blocking
customers from reusing a deleted group's name even though the group
was not visible in the admin. Groups are now hard deleted, relying on
existing django-simple-history records for the audit trail. Group
creation also clears out any legacy soft-deleted row for the same
name/customer so pre-existing zombie rows don't block re-creation.

Co-authored-by: sjasti-sonata-svg <sjasti-sonata-svg@users.noreply.github.com>