Discovery's "Suggested albums" surfaced singles and EPs because
_derive_albums filtered with is_core_release (Album/Single/EP). Add a
discovery-only _album_kind_ok gate that, when discover.albumsOnly (new
config, default true), requires primary_type == "Album" with no secondary
types. is_core_release is untouched — the monitor/scan still use it.
Exposed discover.albumsOnly in the discover config route + a Discovery
settings toggle. Worker + web tests added.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Jonathan