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`

package.json

@@ -88,6 +88,12 @@
     "./features/admin/components": {
       "import": "./dist/features/admin/components/index.js"
     },
+    "./features/media": {
+      "import": "./dist/features/media/index.js"
+    },
+    "./features/media/picker": {
+      "import": "./dist/features/media/components/MediaPicker.client.js"
+    },
     "./features/provider": {
       "import": "./dist/features/provider/index.js"
     },