Add traverse access level for folder navigation without file access

When a permission is set on a nested folder, traverse is automatically
granted on all ancestor folders so users can navigate to it. Traverse
only shows subfolders in listings — files are hidden. This prevents
orphaned permissions where a user has access to a nested folder but
can't reach it.

Hierarchy: traverse < view < edit < admin

packages/backend/src/db/schema/storage.ts

@@ -11,7 +11,7 @@ import {
 import { users } from './users.js'
 import { roles } from './rbac.js'
 
-export const storageFolderAccessEnum = pgEnum('storage_folder_access', ['view', 'edit', 'admin'])
+export const storageFolderAccessEnum = pgEnum('storage_folder_access', ['traverse', 'view', 'edit', 'admin'])
 
 export const storageFolders = pgTable('storage_folder', {
   id: uuid('id').primaryKey().defaultRandom(),