Restructure the core API module to improve clarity, consistency, and
maintainability:
- Introduce `defineApiRoutes()` helper for declarative route definitions
with built-in config validation at startup
- Add `apiSuccess()` / `apiError()` response utilities; enforce their
use across all handlers (core and modules)
- Move auth enforcement to route definitions (`auth: 'public' | 'user' |
'admin'`), removing manual auth checks from handlers
- Extract core routes into `core-routes.js`; router now has no knowledge
of specific features
- Rename `nx-route.js` to `route-handler.js` and update package.json
export accordingly
- Update ARCHITECTURE.md to reflect new API conventions and point to
`src/core/api/README.md` for details
