invoicejob: injbAmount accepts Infinity/-Infinity — DOUBLE column stores them as inf

[CryptoJones]

Problem

app/schemas/invoicejob.schema.js declares injbAmount as
z.coerce.number(). zod's .number() rejects NaN by default but
allows Infinity and -Infinity through.

JSON has no literal for Infinity, but z.coerce.number() happily
turns the string "Infinity" into the float, and the underlying
Sequelize DOUBLE column accepts the value and stores it as inf.
Any downstream consumer doing arithmetic on the column (invoice
totals, aging buckets, CSV exports) then gets contaminated.

This mirrors the cpayAmount bug fixed in #172, on the invoice-line
side. Unlike the payment field, injbAmount legitimately accepts
zero (reference / courtesy lines) and negatives (credits, discounts),
so the appropriate refinement is .finite() only — not a non-zero
gate.

Fix

• Add .finite() to injbAmount via a shared injbAmountField
  validator (matches cpayAmountField pattern from feat(customerpayment): reject zero + non-finite cpayAmount at the schema layer #172).
• Pin in tests:
  • POST with injbAmount: 'Infinity' → 400 (string coerced)
  • POST with injbAmount: '-Infinity' → 400
  • POST with injbAmount: 0 → not 400 (still valid)
  • POST with injbAmount: -50 → not 400 (still valid)

Acceptance

• Schema rejects Infinity / -Infinity (post coerce)
• Schema still accepts zero
• Schema still accepts negatives
• Tests cover all four cases

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