Corporate provisioning: accept employee_id on the bulk CSV path

The manual Add Corporate User form now requires an Employee ID and persists it in normalized form (PR 189, shipped Aug 16 2026). The bulk CSV path at /api/admin/users/corporate/bulk still explicitly rejects an employee_id column and derives keys from the email local part instead. The two paths now behave differently on the same field, which matters ahead of the roughly 8,000-employee bootstrap. Decision needed on whether bulk accepts the column, and if so whether it is required there as it is on the manual form. Source: scoped out of PR 189 deliberately.