URL MaticStory-slug generator for unguessable upload URLs

Note — each slug is a tiny readable story (who does what), with every word chosen for a positive, friendly meaning. Words are drawn with a cryptographic random source — readable for humans, hard to guess for everyone else; entropy is shown above. Digits 0 and 4 are never used. A separator is added automatically after the prefix and before the suffix (skipped when one is already there).