Host hook for core's own uploads (user avatars, branding logos, media selector uploads with a scope):

config :phoenix_kit, :uploads_parent_folder, {MyApp.Media, :parent_for}

called as parent_for(kind, actor_uuid, subject) (or parent_for(kind, actor_uuid)), kind ∈ :avatar | :branding, returning {:ok, folder_uuid} or nil (root, default).

• actor_uuid is whoever is uploading. On the admin user form that is the admin and subject is the user being edited; Auth.update_user_avatar/4 passes the avatar's owner as both.
• The avatar and branding pickers pass the answer to MediaSelectorModal as scope_folder_id, which also scopes browsing to that folder's subtree — files outside it are not offered in those pickers.
• The pickers consult the hook when they open, not on page mount, so a hook that lazily creates its folder does so only when someone uploads.
• An answer that is not a UUID, names no folder, or names a trashed folder is treated as nil, as is a hook that raises or exits.