[n30nex]

If JSON encoding failed, the shared response helper had already committed HTTP 200 and returned an empty body. Encode before writing headers and return the standard structured 500 response on failure. Successful responses keep their JSON content type and trailing newline; network write failures are logged separately.

Validation:

• The new NaN/unsupported-type regression failed before the fix and passes afterward.
• Local build, vet, full Go tests and vulnerability checks passed.
• Full Linux/PostgreSQL CI passed.
• Swagger regenerated with no schema changes.

Closes #254.