fix(server,web): forward-fix CI lint + vitest failures

- gofmt -s on system.go, system_cron.go, api.go
- rename unused r → _ in 3 fetcher_test.go HTTP handlers
- TrackRow +queue test uses /add .* to queue/i (track-aware aria-label from #377)
- /library/artists page test mocks likes + tanstack-query (ArtistCard now embeds LikeButton)

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

internal/playlists/system.go

@@ -82,12 +82,12 @@ const systemMixLength = 25
 // JitterMagnitude is 0 because daily determinism comes from tieBreakHash;
 // any randomness would defeat the within-day stability invariant.
 var systemMixWeights = recommendation.ScoringWeights{
-	BaseWeight:      1.0,
-	LikeBoost:       2.0,
-	RecencyWeight:   1.0,
-	SkipPenalty:     2.0,
-	JitterMagnitude: 0.0,
-	ContextWeight:   0.5,
+	BaseWeight:       1.0,
+	LikeBoost:        2.0,
+	RecencyWeight:    1.0,
+	SkipPenalty:      2.0,
+	JitterMagnitude:  0.0,
+	ContextWeight:    0.5,
 	SimilarityWeight: 1.5,
 }