New src/rbac.py: Role enum, hierarchy, get_user_role(), has_role(),
is_admin(), is_km_admin(), has_dataset_access(), set_user_role().
webapp/auth.py: admin_required + km_admin_required now use DuckDB
roles instead of Linux groups (pwd.getpwnam + sudo/data-ops check).
app/auth/dependencies.py: imports Role from src/rbac.py (single source).
11 RBAC tests passing.
