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
+}

internal/scrobble/threshold_test.go

@@ -0,0 +1,42 @@
+package scrobble
+
+import "testing"
+
+func TestQualifies_NeverPlayed(t *testing.T) {
+	if Qualifies(0, 0) {
+		t.Error("0/0 should not qualify")
+	}
+}
+
+func TestQualifies_AtLeast240Seconds(t *testing.T) {
+	if !Qualifies(240_000, 0.0) {
+		t.Error("240_000 ms played should qualify regardless of ratio")
+	}
+}
+
+func TestQualifies_AtLeastHalfRatio(t *testing.T) {
+	if !Qualifies(0, 0.5) {
+		t.Error("ratio=0.5 should qualify regardless of duration")
+	}
+}
+
+func TestQualifies_BelowBoth(t *testing.T) {
+	if Qualifies(120_000, 0.49) {
+		t.Error("120s + 49% should not qualify")
+	}
+}
+
+func TestQualifies_BoundaryJustUnder(t *testing.T) {
+	if Qualifies(239_999, 0.4999) {
+		t.Error("just-under both thresholds should not qualify")
+	}
+}
+
+func TestQualifies_BoundaryExactly(t *testing.T) {
+	if !Qualifies(240_000, 0.4999) {
+		t.Error("exactly 240s should qualify")
+	}
+	if !Qualifies(239_999, 0.5) {
+		t.Error("exactly 50% should qualify")
+	}
+}