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>

worker/tests/test_discovery.py

@@ -11,6 +11,48 @@ def _artist_rows(conn):
         return cur.fetchall()
 
 
+def _insert_library_item(conn, artist, artist_mbid, album="Owned Album"):
+    with conn.cursor() as cur:
+        cur.execute(
+            'INSERT INTO "LibraryItem" (id, artist, album, path, source, format, "qualityClass", '
+            '"artistMbid", "importedAt") '
+            "VALUES (gen_random_uuid()::text, %s, %s, %s, 'scan', 'FLAC', 2, %s, now())",
+            (artist, album, f"/music/{artist}/{album}", artist_mbid),
+        )
+    conn.commit()
+
+
+def test_seeds_discovery_from_owned_album_artists(conn):
+    # An artist you own an album by but don't follow becomes a discovery seed.
+    _insert_library_item(conn, "Owned Artist", "lib-seed")
+    src = FakeSimilaritySource(similar={"lib-seed": [SimilarArtist("c1", "New Find", 0.9)]})
+    result = run_discovery(conn, [src], FakeMbBrowser(), DiscoveryConfig())
+    assert result.seeds == 1
+    assert [r[0] for r in _artist_rows(conn)] == ["c1"]  # surfaced from the library seed
+    with conn.cursor() as cur:  # library seed throttled via DiscoverySeed, not WatchedArtist
+        cur.execute('SELECT "lastDiscoveredAt" IS NOT NULL FROM "DiscoverySeed" WHERE mbid = %s', ("lib-seed",))
+        assert cur.fetchone()[0] is True
+
+
+def test_library_seeding_can_be_disabled(conn):
+    _insert_library_item(conn, "Owned Artist", "lib-seed")
+    src = FakeSimilaritySource(similar={"lib-seed": [SimilarArtist("c1", "New Find", 0.9)]})
+    result = run_discovery(conn, [src], FakeMbBrowser(), DiscoveryConfig(seed_from_library=False))
+    assert result.seeds == 0 and _artist_rows(conn) == []  # library not seeded
+
+
+def test_owned_artist_is_not_suggested_as_a_candidate(conn):
+    # You own an album by "c1"; a followed seed finds c1 similar. c1 must not be suggested back.
+    insert_watched_artist(conn, mbid="s1", name="Seed")
+    _insert_library_item(conn, "Owned", "c1")
+    src = FakeSimilaritySource(similar={
+        "s1": [SimilarArtist("c1", "Owned", 0.9), SimilarArtist("c2", "New", 0.8)],
+        "c1": [],  # the library seed itself finds nothing new here
+    })
+    run_discovery(conn, [src], FakeMbBrowser(), DiscoveryConfig())
+    assert [r[0] for r in _artist_rows(conn)] == ["c2"]  # c1 excluded (already owned)
+
+
 def test_aggregates_scores_across_seeds(conn):
     insert_watched_artist(conn, mbid="s1", name="Seed One")
     insert_watched_artist(conn, mbid="s2", name="Seed Two")