API self-test

Exercises every endpoint this server exposes against a known public file (mzspec:MSV000084951:AH22) and reports what passed. Use it to confirm a deployment works. The first run downloads and parses that file if it is not already cached, so it can take a minute; later runs are fast. Everything here is read-only apart from one same-origin short URL that expires after an hour. Append ?run=1 to start automatically.