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/test/setup.ts

@@ -17,6 +17,7 @@ beforeEach(async () => {
   await prisma.watchedArtist.deleteMany();
   await prisma.discoverySuggestion.deleteMany();
   await prisma.discoverySeedContribution.deleteMany();
+  await prisma.discoverySeed.deleteMany();
   await prisma.config.deleteMany();
   await prisma.apiCache.deleteMany(); // read-through cache must not leak across tests
   await prisma.unmatchedAlbum.deleteMany();