Add comprehensive HR demo seeder with full employee/trainer lifecycle data

Covers: 11 employees (6 staff + 4 trainer-employees + 1 resigned), all 12 trainers
with bios/sports/qualifications/availability, 3 months of compensation history,
2 closed payroll periods with payslips, advances in all states, rate history.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>