- is_paid now = remaining_rials == 0 (was payment_method != pending), so adding
a service/package after settlement correctly flips the session back to debtor.
- updateSessionServices recomputes the cached settlement (payment_method/paid_at)
after totals change.
- addSessionPayment now records a 'create' audit entry (actor, amount, time).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>