Add role-based access control with hierarchy: super_admin > team_admin >
engineer > viewer. Adds is_super_admin boolean to User model (migration 010),
centralized backend permissions module, frontend usePermissions hook, and
UI enforcement (conditional Create/Edit buttons, editor redirect for viewers,
role badge in header). All endpoint admin checks updated from role=="admin"
to is_super_admin.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
