Each subscription row now independently stores:
- receipt_number: denormalized from receipts table for quick audit access
- paid_by: FK to employees — who processed the payment
- paid_at: already existed

Migration adds columns and backfills existing paid rows from payments/receipts.
View shows "بيانات السداد" column with date, receipt number, and employee name.
Model query now JOINs employees for paid_by_name display.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>