perf(similarity): worker batch 5→25; track-MBID backfill uncapped

- similarity.Worker batch 5→25 (tracks AND artists per 1h tick). At
  5/h a freshly-scrobbled library took days to build a usable
  similarity pool; 25/h converges in hours, still well under
  ListenBrainz rate limits (429 aborts the tick).
- scanrun Stage 2b track backfill now runs unbounded (-1) instead of
  reusing the album backfill's 5000 staged cap. It's a one-time
  whole-library heal with no progress UI; a cap just left tracks.mbid
  partially NULL (3634/18056 after one pass) until several future
  scans caught up, re-reading untagged files each time. One uncapped
  pass converges; later scans only re-read the remaining NULL rows.
  Album backfill keeps its 5000 cap (it has staged scan_runs UX).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/similarity/worker.go

@@ -34,14 +34,16 @@ type Worker struct {
 }
 
 // NewWorker constructs a worker with production defaults: 1h tick,
-// batch=5, topK=20.
+// batch=25, topK=20. batch is tracks AND artists processed per tick;
+// at 25/h a freshly-played library converges in hours, not days, while
+// staying well under ListenBrainz rate limits (429s abort the tick).
 func NewWorker(pool *pgxpool.Pool, client *listenbrainz.Client, logger *slog.Logger) *Worker {
 	return &Worker{
 		pool:   pool,
 		client: client,
 		logger: logger,
 		tick:   1 * time.Hour,
-		batch:  5,
+		batch:  25,
 		topK:   20,
 	}
 }