feat(scrobble): add pure Qualifies threshold function

internal/scrobble/threshold.go

@@ -0,0 +1,15 @@
+// Package scrobble owns the outbound-scrobble pipeline: threshold detection,
+// the queue write path, and the worker goroutine. The HTTP client is in
+// internal/scrobble/listenbrainz; this package consumes it.
+package scrobble
+
+// Qualifies returns true if a closed play_event meets ListenBrainz's
+// recommended scrobble threshold: ≥240 seconds played OR ≥50% of the
+// track length played.
+//
+// The two thresholds are deliberately separate from M2's skip-detection
+// thresholds (which serve a different purpose — internal engine signal
+// for the scoring formula's skip penalty).
+func Qualifies(durationPlayedMs int, completionRatio float64) bool {
+	return durationPlayedMs >= 240_000 || completionRatio >= 0.5
+}