feat(modules): add external module registration and defineModule support

- Add `./modules/define` export path pointing to `defineModule.js`
- Implement `registerExternalModules()` to handle modules passed via `zen.config.js`, with env var gating (`ZEN_MODULE_<NAME>=true`)
- Extract `buildAdminConfig()` helper to consolidate admin navigation/page config building
- Refactor `loadModuleConfig()` to use `buildAdminConfig()` and simplify public routes check
- Improve `initializeModuleTables()` to gracefully skip modules without `db.js` instead of erroring
- Update module discovery JSDoc to reflect external module registration support

package.json

@@ -135,6 +135,9 @@
     "./modules": {
       "import": "./dist/modules/index.js"
     },
+    "./modules/define": {
+      "import": "./dist/core/modules/defineModule.js"
+    },
     "./modules/pages": {
       "import": "./dist/modules/pages.js"
     },