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`

.env.example

@@ -55,4 +55,7 @@ ZEN_PUBLIC_LOGO_URL=
 NEXT_TELEMETRY_DISABLED=1
 
 # DEVKIT (developer tools)
-ZEN_DEVKIT=false
+ZEN_DEVKIT=false
+
+# MEDIA (gestionnaire de médias CMS — images, PDFs, vidéos attachés au contenu du site)
+ZEN_MEDIA=false