customerpayment: cpayAmount accepts 0 and Infinity — both operator/data-quality bugs

[CryptoJones]

Problem

app/schemas/customerpayment.schema.js declares the amount field as:

cpayAmount: z.coerce.number(),

z.coerce.number() accepts every value Number() would produce — including:

• 0 — a $0 payment recorded against a customer ledger has no business
  meaning. Always operator error.
• Infinity / -Infinity — zod's .number() rejects NaN by default
  but lets the infinities through. The cpayAmount column is a Sequelize
  DOUBLE which happily stores inf, and any consumer doing arithmetic
  (totals, aging buckets, CSV exports) gets contaminated thereafter.

Negative values are valid — some operators model refunds that way.

Fix

Extract the amount validator to a named cpayAmountField so the create
and update bodies share one definition, and chain:

.finite({ message: 'cpayAmount must be a finite number.' })
.refine((n) => n !== 0, { message: 'cpayAmount must not be zero.' })

Pin the new behavior in tests/api/customerpayment.test.js:

• POST with cpayAmount: 0 → 400
• POST with cpayAmount: -50 → not 400 (refund flow stays valid)
• PATCH with cpayAmount: 0 → 400

Acceptance

• Schema rejects cpayAmount: 0
• Schema rejects Infinity / -Infinity
• Schema still accepts negative values
• Tests cover all three cases

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