refactor(api): refactor API module with route definitions and response utilities

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

package.json

@@ -83,7 +83,7 @@
       "import": "./dist/core/api/index.js"
     },
     "./zen/api": {
-      "import": "./dist/core/api/nx-route.js"
+      "import": "./dist/core/api/route-handler.js"
     },
     "./database": {
       "import": "./dist/core/database/index.js"