fix(worker): evict removed-seed contributions + clear scan worklist on abandon

Two row-leak bugs found in whole-branch review:

- discovery.run_discovery now prunes DiscoverySeedContribution rows whose
  seedMbid is no longer a WatchedArtist (unfollow/removal has no FK cascade)
  and recomputes the touched candidates' scores this sweep, instead of
  letting a dead seed's contribution inflate the aggregate forever.

- maybe_run_scan's exception handler around scan_chunk now calls
  clear_worklist and clears scan.id, matching the drain path. Previously
  an exception mid-chunk left the already-done rows plus the remaining
  worklist for that scan_id orphaned forever, since the next scan mints a
  fresh uuid. Also clear scan.id in the build_worklist failure handler for
  consistency.

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

worker/tests/test_discovery.py

@@ -165,6 +165,40 @@ def test_seed_dropoff_lowers_seedcount(conn):
     assert _artist_score(conn, "c1") == (0.5, 1)  # only s2 remains
 
 
+def test_evicts_contributions_from_removed_seed(conn):
+    # Two live seeds contribute to c1 -> 1.1/2. Then s1 is unfollowed (deleted from
+    # WatchedArtist, simulating DELETE /api/artists/[id]) without any FK cascade on
+    # DiscoverySeedContribution. The next sweep must evict s1's orphaned contribution
+    # rather than leaving it to inflate c1's score forever.
+    insert_watched_artist(conn, mbid="s1", name="Seed One",
+                          last_discovered_sql="now() - interval '200 hours'")
+    insert_watched_artist(conn, mbid="s2", name="Seed Two",
+                          last_discovered_sql="now() - interval '200 hours'")
+    src = FakeSimilaritySource(similar={
+        "s1": [SimilarArtist("c1", "Shared", 0.6)],
+        "s2": [SimilarArtist("c1", "Shared", 0.5)],
+    })
+    run_discovery(conn, [src], FakeMbBrowser(), DiscoveryConfig())
+    assert _artist_score(conn, "c1") == (1.1, 2)
+
+    # Unfollow s1 (no FK cascade on DiscoverySeedContribution).
+    with conn.cursor() as cur:
+        cur.execute('DELETE FROM "WatchedArtist" WHERE mbid = %s', ("s1",))
+    conn.commit()
+
+    # Make s2 due again and sweep; s1 can no longer be a seed since it's gone.
+    with conn.cursor() as cur:
+        cur.execute('UPDATE "WatchedArtist" SET "lastDiscoveredAt" = now() - interval \'200 hours\' WHERE mbid = %s', ("s2",))
+    conn.commit()
+    src2 = FakeSimilaritySource(similar={"s2": [SimilarArtist("c1", "Shared", 0.5)]})
+    run_discovery(conn, [src2], FakeMbBrowser(), DiscoveryConfig())
+
+    assert _artist_score(conn, "c1") == (0.5, 1)  # s1's stale 0.6 evicted, not summed
+    with conn.cursor() as cur:
+        cur.execute('SELECT count(*) FROM "DiscoverySeedContribution" WHERE "seedMbid" = %s', ("s1",))
+        assert cur.fetchone()[0] == 0
+
+
 def test_run_discovery_processes_at_most_chunk_size_seeds(conn):
     from lyra_worker.discovery import DiscoveryConfig, run_discovery
     from lyra_worker.adapters.fakes import FakeMbBrowser, FakeSimilaritySource