Installment status change notification

Webhook endpoint called by QI Tech when an installment status changes. This notification is used to keep the BNPL system in sync with the financial institution.

Common status transitions:

• PENDING -> PAID: Normal on-time payment
• PENDING -> OVERDUE_GRACE: Payment missed past due date (no fines yet, BNPL limit zeroed)
• OVERDUE_GRACE -> OVERDUE_PENALTY: Grace period expired, fines and interest now apply
• PENDING -> PAID_EARLY: Payment before due date
• OVERDUE_PENALTY -> PAID_OVERDUE: Late payment with fine/interest
• OVERDUE_PENALTY -> PAID_PARTIAL_OVERDUE: Partial late payment
• OVERDUE_GRACE -> PAID: Payment received during grace period (no fines)
• PENDING -> WAITING_PAYMENT: Payment initiated but not confirmed