Faster run exports

Improved

• Run exports download at once. Exporting a run's findings as CSV, JSON or Markdown now streams the file as soon as you choose the format, instead of preparing it first. Large runs export too: the 64 MiB limit is gone, and a run still exports at most 5,000 findings. See Export run findings.
• Run exports are named after the run's number. The file is now v12-<N>.csv, v12-<N>.json or v12-<N>.md, where N is the number shown as Run #N on the run's page.