Added response.go, consistent error response

[Howard-nolan]

Change Summary:

• Added a response.go helper to centralize JSON response formatting.
• Introduced a consistent JSON error shape used across all HTTP handlers.
• Updated handlers to use shared helpers instead of writing responses inline.

How It Works:

• response.go has WriteJSON and WriteError functions

Additional Notes (optional):

• Existing error responses have changed shape; any clients depending on the old error body format may need to be updated.
• Follow-up: we can extend ErrorResponse with request IDs / correlation IDs or more structured error codes if needed.