Update invoice status

Transitions an invoice to a new status. Not all transitions are valid — the API enforces a state machine.

Valid transitions:

• draft → sent, cancelled
• sent → paid, partial, overdue, cancelled, written_off
• partial → paid, overdue, cancelled, written_off
• overdue → paid, partial, cancelled, written_off

Terminal states (paid, cancelled, written_off) cannot be transitioned further.

For payment-driven transitions, use the Record Payment endpoint instead — it automatically sets the status to paid or partial based on the cumulative amount.