feat: handle channel hangup request

This happens when the far end is ringing but has not accepted the
call and instead declines it. In such cases the originating channel will
be terminated as well.

internal/serve/serve.go

@@ -90,6 +90,8 @@ func serve(ctx context.Context, conn *websocket.Conn, debug bool) int {
 		switch evt.Type {
 		case "StasisStart":
 			handleStart(payload)
+		case "ChannelHangupRequest":
+			fallthrough
 		case "StasisEnd":
 			handleEnd(payload)
 		default: