Fix HR seeder: create branches/people/trainers from scratch, idempotent cleanup

Production had no trainers/employees and only 1 branch. Rewrote seeder to be
fully self-contained: creates 2 new branches, 19 people, 11 employees, 12 trainers,
then populates all HR tables. Cleans up prior data on re-run.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>