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": "",

web/src/app/settings/settings-form.tsx

@@ -395,6 +395,8 @@ export function SettingsForm() {
               <b> Similar per seed</b> — candidates pulled per artist. <b>Albums per artist</b> —
               releases surfaced per suggested artist. <b>Min score</b> — drops weak suggestions.{" "}
               <b>Albums only</b> — suggest full-length albums only, hiding singles and EPs.{" "}
+              <b>Seed from library</b> — also find artists similar to ones you own albums by, not
+              just the artists you follow.{" "}
               <b>ListenBrainz base URL</b> — leave blank for the default endpoint.
             </HelpButton>
           </div>
@@ -414,6 +416,14 @@ export function SettingsForm() {
             />
             Albums only (hide singles &amp; EPs)
           </label>
+          <label className="toggle" style={{ margin: "6px 0 0" }}>
+            <input
+              type="checkbox"
+              checked={discoverCfg["discover.seedFromLibrary"] !== "false"}
+              onChange={(e) => setCfg("discover.seedFromLibrary", e.target.checked ? "true" : "false")}
+            />
+            Seed from library (similar to albums you own)
+          </label>
           <label className="field">
             <span>ListenBrainz base URL (blank = default)</span>
             <input value={discoverCfg["discover.listenBrainzUrl"] ?? ""}

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();