feat(media): add media management feature module

- add `ZEN_MEDIA` env flag and document it in `.env.example`
- add media schema, server routes, and API handlers (`api.server.js`, `routes.server.js`, `schema.server.js`)
- add `MediaPage`, `MediaGrid`, `MediaFilters`, and `MediaPicker` client components
- expose `@zen/core/features/media` and `@zen/core/features/media/picker` package exports
- register media navigation and permissions; wire module into `init.js`
- document media API, client picker usage, and boundary rules in `MODULES.md` and `ARCHITECTURE.md`
- add `src/features/media/README.md`

src/features/admin/AdminPage.client.js

@@ -9,6 +9,7 @@ import './pages/SettingsPage.client.js';
 import './pages/ConfirmEmailChangePage.client.js';
 import './widgets/index.client.js';
 import './devkit/DevkitPage.client.js';
+import '../media/pages/MediaPage.client.js';
 
 export default function AdminPageClient({ params, user, widgetData, appConfig, devkitEnabled }) {
   const parts = params?.admin || [];