Add activity history clearing

internal/queue/queue.go

@@ -151,6 +151,15 @@ func (q *Queue) UpdateActive(id string, update func(*model.Run)) bool {
 	return true
 }
 
+func (q *Queue) ClearHistory() int {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	count := len(q.history)
+	q.history = nil
+	q.emitLocked()
+	return count
+}
+
 func (q *Queue) snapshotLocked() []model.Run {
 	result := append([]model.Run{}, q.history...)
 	if q.active != nil {