feat/Add Invoice Validation Schema

[Oluwatos94]

Summary

• Added validateInvoicePayload() to src/utils/validators.js with strict validation for amount, dueDate, buyer, seller, and currency (ISO 4217)

• Integrated payload validation into POST /api/invoices in app.js returns 400 with all errors collected in a single pass - Wired up GET /api/invoices to use invoiceService.getInvoices with validateInvoiceQueryParams (previously a
  placeholder)

• Fixed broken src/index.js from a prior merge conflict (duplicate imports, missing AppError, broken PATCH restore handler)

  Test plan

• Run npm test 73 tests pass across index.test.js, app.test.js, invoice.api.test.js

  • validators.js has 100% statement/line/function coverage and 100% branch coverage
  • POST /api/invoices returns 400 for: missing fields, zero/negative amount, wrong date format, empty buyer/seller, unsupported currency
  • POST /api/invoices returns 201 for a fully valid payload
  • All errors are collected and returned together (not fail-fast)

  Security notes

  • Currency validated against an explicit ISO 4217 allowlist no arbitrary string passthrough
  • Buyer/seller trimmed to prevent whitespace-only values
  • Amount checked for finiteness to reject Infinity and NaN

closes #2

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