fix TrackUsage and unused import

internal/handler/keys.go

@@ -35,6 +35,12 @@ func save() {
 	os.WriteFile(p, b, 0644)
 }
 
+func TrackUsage(k string, t int) {
+	if k == "free" { return }
+	s.mu.Lock(); defer s.mu.Unlock()
+	if kd, ok := s.Keys[k]; ok { kd.Tokens += t; save() }
+}
+
 func CheckAndTrack(k string, t int) (bool, string) {
 	if k == "free" { return true, "" }
 	s.mu.Lock(); defer s.mu.Unlock()