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/library/scanrun.go

@@ -156,10 +156,16 @@ func RunScan(
 
 	// Stage 2b: track recording-MBID backfill. Unblocks the ListenBrainz
 	// similarity pipeline (gated on tracks.mbid IS NOT NULL). Log-only
-	// progress — no scan_runs jsonb column to avoid a schema addition;
-	// it's idempotent and reruns each scan until every file is healed.
+	// progress — no scan_runs jsonb column to avoid a schema addition.
+	//
+	// Unbounded (-1), unlike the album backfill's 5000 staged cap: this
+	// is a one-time whole-library heal with no progress UI, and a cap
+	// just means tracks.mbid stays partially NULL until N future scans
+	// catch up (re-reading untagged files wastefully each pass). One
+	// uncapped pass converges; subsequent scans only re-read the
+	// remaining NULL (genuinely untagged) rows, which is cheap.
 	if cfg.BackfillCap != 0 {
-		if _, tberr := BackfillTrackMBIDs(ctx, pool, logger, cfg.BackfillCap, nil); tberr != nil {
+		if _, tberr := BackfillTrackMBIDs(ctx, pool, logger, -1, nil); tberr != nil {
 			captureErr("track_mbid_backfill", tberr)
 		}
 	}