Very nice to see this up and running Stas!. I don't see Wikimedia Commons at https://noc.wikimedia.org/conf/categories-rdf.dblist . Could you please add it? It's the wiki I most often query in Petscan.
Topic on Talk:Wikidata Query Service/Categories
I've not added commons yet to the list because commons category list is huge, so I want to make sure the system works smoothly first. After this is ensured, we can add commons too.