feat(role): Add the SUPERADMIN role

[ebouchut]

Summary

Add the SUPERADMIN role to the platform.

Context

The initial roles seed (Liquibase migration V*-seed-roles.sql) intentionally
seeds only STUDENT, INSTRUCTOR, and ADMIN (YAGNI). SUPERADMIN is the top
of the role hierarchy described in #39 but is deferred until it is actually
needed.

Scope

• Seed the SUPERADMIN role via a new Liquibase migration (append-only; do not edit the existing seed).
• Define and enforce its permissions (see the hierarchy and permissions in 🔵 User roles and permissions #39).
• Decide how a SUPERADMIN account is bootstrapped (it is not created through self-registration).

Part of the roles and permissions epic #39.