- Remove non-existent facility_unit_id from sa_groups insert in
  enrollByCoach, add required code column with auto-generated value
- Guard schedule copy when no source group exists
- Fix old() helper usage for array fields (additional_coaches) by
  reading directly from session _old_input instead of old() which
  only returns strings
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>