Finance data model

• Invoice optionally links Company, Contact, Lead, Opportunity, Quote, and Document. It stores lifecycle state, commercial context, and calculated totals.
• InvoiceItem belongs to one invoice and stores quantity, unit price, and calculated line total.
• Payment belongs to an invoice and optionally a schedule row. Its status controls whether it affects balances.
• Receipt has a one-to-one relationship with a confirmed payment.
• Expense records an operational outgoing with category and approval state.
• PaymentSchedule belongs to an invoice and summarizes one installment's due, paid, and balance values.

The existing ActivityLog cleanly supports finance events, so no parallel FinanceActivity model was added. Decimal database columns use two-place monetary precision; reusable calculation rules live in @submicron/finance-engine.