Tags for every shared demo, also old ones

Pages shared on a demo site before it had tags can now all be tagged in one go, including pages whose demo files were deleted on this machine to free space. Obeya tags up to twenty pages at a time: a short session picks their tags from title and text, and one deploy writes them onto the pages, around the video the site already holds. A backfill of a few hundred pages takes minutes, and the overview's filters then cover the whole site.