* feat(assets): add user custom favorite folders
- Add FavoriteFolder model (per-user, nested via self parent FK)
- Add nullable folder FK on FavoriteAsset, allow one asset in multiple folders
- Add FavoriteFolderViewSet and folder filter on FavoriteAssetViewSet
- Serializer outputs asset_info for building connectable tree nodes
- Register favorite-folders route; exclude favoritefolder from rbac/audit
* feat(assets): register favorite-folders route and add migration
- Register favorite-folders router endpoint
- Add migration 0020 creating FavoriteFolder and folder FK on FavoriteAsset
* feat(rbac): exclude favoritefolder from permission management
Keep favoritefolder consistent with favoriteasset (user-owned, not RBAC managed)
* feat(audits): exclude FavoriteFolder from operate log
