Set dc callback redirect status to 301 for better SEO

@leevan please note that the callback URL should not be indexed at all by any search engine. There is no redirection if you click a link to the callback URL in a search engine. If you have multiple nodes protected by doccheck, redirection depends on the last node visited. Since the response is not cached, it is not considered permanent. So the audit result is probably the only thing that would be improved, but I'm open to other uses for the 302 response which could be configurable in admin.