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/media/navigation.js

@@ -0,0 +1,25 @@
+/**
+ * Media Feature — Admin navigation.
+ *
+ * Side effect : enregistre la section "Contenu" et l'item "Médias" dans la
+ * sidebar admin si le module est activé via ZEN_MEDIA=true.
+ */
+
+// Import direct depuis le registre pour éviter une dépendance circulaire :
+// admin/navigation.js → media/navigation.js → admin barrel → admin/navigation.js
+import { registerNavSection, registerNavItem } from '../admin/registry.js';
+import { isMediaEnabled } from '@zen/core/shared/config';
+import { MEDIA_PERMISSIONS } from './permissions.js';
+
+if (isMediaEnabled()) {
+  registerNavSection({ id: 'content', title: 'Contenu', icon: 'File02Icon', order: 25 });
+  registerNavItem({
+    id: 'media',
+    label: 'Médias',
+    icon: 'Image01Icon',
+    href: '/admin/media',
+    sectionId: 'content',
+    order: 10,
+    permission: MEDIA_PERMISSIONS.VIEW,
+  });
+}