Alembic migration 86c7b1d6a710 adds is_admin BOOLEAN DEFAULT FALSE
to the roles table. Fixes login crash after M-3 change:
psycopg2.errors.UndefinedColumn: column roles.is_admin does not exist
Also adds _ensure_roles_is_admin_column() as an additive migration
safety net for environments that don't run alembic automatically.
To apply: cd backend && source .venv/bin/activate && alembic upgrade head