feat(discover): suggest full albums only (filter singles/EPs)

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>

web/src/app/api/discover/config/route.ts

@@ -5,6 +5,7 @@ const DEFAULTS: Record<string, string> = {
   "discover.intervalHours": "168",
   "discover.similarPerSeed": "20",
   "discover.albumsPerArtist": "1",
+  "discover.albumsOnly": "true",
   "discover.minScore": "0",
   "discover.chunkSize": "5",
   "discover.listenBrainzUrl": "",