Add Advance Payment type to payroll adjustments. Admins can give
workers partial advances against earned wages — each advance creates
a payslip sent to Spark immediately, while the remaining balance stays
on the dashboard. Multiple advances allowed. Final payment deducts
all advances automatically.
Also refactored type-grouping logic into ADDITIVE_TYPES/DEDUCTIVE_TYPES
constants (was duplicated 15 times across views).
Includes migration 0016 (choices-only, run: python manage.py migrate)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
