Remove hub mode (step 2/2): delete dead hub routes & views
Remove routes/hub.js, routes/artists.js (+ their server.js mounts/imports/redirect)
and the hub-home + artists-directory views. All were unreachable after step 1
(tenancy can no longer be 'hub'). Shared multi-site infra (getPrimarySite,
site_members, canAdminSite) stays — solo uses it too.
Co-Authored-By: Claude <noreply@…>