Backend: delete custom role #581

Add an API operation to delete a custom role, removing its Casbin p rows and definition/relationship rows (per the persistence model). The UI drives two deletion paths the contract must support (define in #469):

1. Delete + revoke ("Delete anyway"): delete the role and unassign all its members.
2. Reassign + delete ("Reassign & Delete"): first reassign all members to a chosen replacement role, then delete the original.

Superuser-only. Standard roles are not deletable (ADR 0021).

Acceptance

• Deleting a custom role removes its definition, role-permission and policy rows and its assignments.
• Both the revoke-all and reassign-then-delete paths implemented and tested.
• Audit recorded for the deletion and the member changes.