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/main.py

@@ -76,6 +76,7 @@ def maybe_run_scan(conn, resolver, probe, browser, config, dest_root: str = DEST
         except Exception as e:  # a scan error must never kill the worker
             print(f"worker: library scan worklist build failed: {e}", flush=True)
             conn.rollback()
+            _set_config(conn, "scan.id", "")
             _set_config(conn, "scan.inProgress", "false")
             _set_config(conn, "scan.requested", "false")
             return
@@ -91,7 +92,9 @@ def maybe_run_scan(conn, resolver, probe, browser, config, dest_root: str = DEST
     except Exception as e:  # a scan error must never kill the worker
         print(f"worker: library scan chunk failed: {e}", flush=True)
         conn.rollback()
-        _set_config(conn, "scan.inProgress", "false")
+        clear_worklist(conn, scan_id)
+        _set_config(conn, "scan.id", "")
+        _set_config(conn, "scan.inProgress", "false")  # abandon the scan; a new request restarts it
         _set_config(conn, "scan.requested", "false")
         return
     imported_total = imported_so_far + imp