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/lyra_worker/discovery.py

@@ -225,7 +225,17 @@ def run_discovery(conn: psycopg.Connection, sources: list[SimilaritySource],
     followed = _followed_mbids(conn)
     live = _healthy_sources(sources)
 
-    affected: set[str] = set()
+    # Evict contributions from seeds no longer followed — unfollow/removal has no
+    # FK cascade, so without this a removed seed's score would linger and inflate
+    # the aggregate forever. Seed `affected` with the touched candidates so their
+    # suggestion scores are recomputed from the remaining live seeds this sweep.
+    with conn.cursor() as cur:
+        cur.execute('SELECT DISTINCT "candidateMbid" FROM "DiscoverySeedContribution" '
+                    'WHERE "seedMbid" NOT IN (SELECT mbid FROM "WatchedArtist")')
+        affected = {r[0] for r in cur.fetchall()}
+        cur.execute('DELETE FROM "DiscoverySeedContribution" '
+                    'WHERE "seedMbid" NOT IN (SELECT mbid FROM "WatchedArtist")')
+
     for seed_mbid, _seed_name in seeds:
         affected |= _record_contributions(conn, seed_mbid, live, followed, cfg)