Decimal amounts stored as integers (cents), never floats

IEEE 754 floating-point arithmetic cannot represent most decimal fractions exactly. Storing 19.99 as a float in PostgreSQL or MongoDB introduces rounding errors that compound across thousands of transactions — a classic fintech data-integrity failure cited under CWE-681 (Incorrect Conversion Between Numeric Types) and CWE-682 (Incorrect Calculation). At scale, accumulated errors produce off-by-one-cent discrepancies in statements, reconciliation failures, and potential regulatory exposure under GAAP precision requirements. Integer cent storage eliminates the issue entirely: 1999 is exact in every language and database type system.

Change all monetary database columns to integer types and add an explicit cents-conversion step in the API layer before any write. Reference prisma/schema.prisma for schema changes.

// prisma/schema.prisma
model Transaction {
  id          String @id @default(cuid())
  amountCents Int    // never Float or Decimal
  currency    String
}

// src/lib/currency.ts
export function toCents(decimalString: string): number {
  return Math.round(parseFloat(decimalString) * 100);
}
export function fromCents(cents: number): string {
  return (cents / 100).toFixed(2);
}