Allow accounts to record partial/advance payments against a PO before
full delivery. A new PARTIALLY_PAID status tracks in-progress payment;
paidAmount accumulates across multiple markPaid calls. PO only closes
when both paidAmount >= totalAmount AND all line items are delivered.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
PO_SUBMITTED: managers only, submitter no longer copied
PAYMENT_SENT: submitter only (it is a receipt prompt, not a manager action)
PARTIAL_RECEIPT_CONFIRMED: managers now notified via new event type
RECEIPT_CONFIRMED: unchanged (managers + accounts)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
