refactor: centralize routing error taxonomy

[telemarkdigital-publisher]

Closes #554

Summary

• Added a typed ApiError taxonomy with one code-to-status/message map and a shared apiErrorHandler.
• Routed parser errors, domain errors, not-found, conflict, and unexpected failures through the same safe formatter.
• Preserved requestId correlation and kept error as a compatibility alias while adding the requested code field.
• Updated API/architecture docs for the new canonical envelope.

Verification

• npm run lint (passes; existing warnings only)
• npm test -- --runInBand
• npm run build