Introduce a Money value object

Money is represented as number amount and string currency across refunds, credits, and invoices. Value Object: model a concept by its values, invariants, equality, and behavior rather than identity. The common trap is changing every API and database field at once, mixing a domain refactor with a migration. Define invariants Money stores integer minor units and uppercase ISO currency; constructor rejects unknown currency and non-integer minor units. A value object earns its keep by preventing bad combinations at creation. Add behavior Implement add, subtract, isZero, and format, with cross-currency operations rejected. Move repeated call-site rules into the concept itself. Refactor…

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