go vet broke because playlists_{discover,foryou}_refresh_test.go
referenced the handlers/types deleted in R2 (d67c0de). Consolidated
into playlists_system_test.go covering the generic
/playlists/system/{kind}/{refresh} endpoint: for_you + discover
200/shape, non-singleton & unknown kind → 404, no-auth → 401.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
