Reject non-positive payment amounts

[13415887994]

Summary

• Reject non-finite and non-positive payment intent amounts with a 400 error.
• Allow the API error handler to return intentional client error status codes instead of converting every thrown error to 500.
• Add service-level regression tests for invalid and valid payment amounts.

Closes #4276

Tests

• node --test apps/api/src/tests/paymentService.test.js