docs/refactor: rename getModuleMetadata and update route auth format

- Rename `getModuleMetadata` to `getModuleMetadataGenerator` in registry,
  index, and client exports to clarify its purpose (returns a generator
  function, not a metadata object)
- Add new `getModuleMetadata` and `getMetadataGenerator` exports from
  `modules.metadata.js` for server-side metadata object retrieval
- Update route auth format in docs from `requireAuth`/`requireAdmin`
  flags to a single `auth` field with values: `'admin'`, `'user'`,
  or `'public'`
- Fix `isModuleEnabledInEnv` to replace hyphens with underscores in
  env var names (e.g. `my-module` → `ZEN_MODULE_MY_MODULE`)
- Replace `useState` initializer in `ZenProvider` with `useRef` guard
  to avoid React strict mode double-invocation issues

docs/modules/INTERNAL_MODULE.md

@@ -132,17 +132,14 @@ async function handleList(request) {
 
 export default {
   routes: [
-    {
-      path: 'mon-module/list',
-      method: 'GET',
-      handler: handleList,
-      requireAuth: true,
-      requireAdmin: false,
-    },
+    { path: '/admin/mon-module/list', method: 'GET', handler: handleList, auth: 'admin' },
+    { path: '/mon-module/list',       method: 'GET', handler: handleList, auth: 'public' },
   ],
 };
 ```
 
+Valeurs acceptées pour `auth` : `'admin'` (JWT admin requis), `'user'` (JWT utilisateur requis), `'public'` (aucune auth).
+
 ---
 
 ## cron.config.js