Invoice status change notification

Webhook endpoint for receiving invoice (fatura) status change notifications. This is triggered when an invoice transitions between states.

Common status transitions:

• OPEN -> PAID: Invoice fully paid
• OPEN -> PARTIALLY_PAID: Partial payment received
• OPEN -> OVERDUE_GRACE: Invoice past due date (no fines yet, BNPL limit zeroed)
• OVERDUE_GRACE -> OVERDUE_PENALTY: Grace period expired, fines and interest now apply
• PARTIALLY_PAID -> PAID: Remaining balance paid
• OVERDUE_GRACE -> PAID: Payment received during grace period (no fines)
• OVERDUE_PENALTY -> PAID: Late payment received (with fines)