feat(discover): also seed discovery from owned-album artists

Discovery seeded only from followed artists. Now it also seeds from artists
you own albums by (LibraryItem.artistMbid, captured since the MBID own-state
work) but don't follow — so your library shapes suggestions, not just your
follows.

- New DiscoverySeed throttle table (migration add_discovery_seed) mirroring
  WatchedArtist.lastDiscoveredAt, so each library-derived seed is swept once
  per interval (WatchedArtist seeds keep their own throttle).
- _seed_artists merges both seed sources least-recently-swept first, capped at
  chunk_size; run_discovery stamps the right throttle per seed and evicts
  contributions from seeds that are neither followed nor owned.
- Artists you already have (followed OR owned) are excluded from the suggestion
  candidates — discovery surfaces only genuinely new artists.
- discover.seedFromLibrary config (default true) + a Discovery settings toggle.

NOTE: existing LibraryItem rows have null artistMbid (populated on new
import/scan), so a re-scan is needed before old albums seed discovery.
worker 236, web 187 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

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