test(api): Add contract tests for API response schemas (#36)

[Sam-Rytech]

Description

This PR implements the API response schema contract tests to prevent breaking changes and ensure stable client expectations, as requested in issue #36.

Changes Made

• Fixed a syntax error in src/app.js that was preventing the application from running.
• Added missing knex dependency required for the test environment.
• Created tests/contract/api-schemas.test.js using supertest.
• Implemented schema validations for:
  • GET /health
  • GET /api/invoices
  • POST /api/invoices

Closes #36

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[Sam-Rytech]

@mikewheeleer PR opened

[Sam-Rytech]

@mikewheeleer