feat(api): register dependents section with per-row documents

Add the employee DependentsSection (repeater rows with name,
relationship_type, id_number, job, phone; min one row when touched)
and move EmployeeService onto the shared SectionRegistry. Each
dependent carries per-row identity documents through the dynamic
placement contract: national card front/back and birth certificate
pages keyed by row field key, relationship-aware labels and ascii
slugs, and completion enforcement via EnforcesRowDocuments. Covered
by section unit and document requirement feature tests.