Add a scheduled check for dead external links in the docs and explainers #1127

Where: a new scripts/check_external_links.py, a scheduled workflow under .github/workflows/, Makefile.

Current gap: scripts/check_broken_links.py checks only internal links. The 108 explainers carry about 200 external references (papers, court cases, regulators, news), and four already return 404 (see the open issue). Nothing notices when a source moves. A check on every PR would be slow and flaky (many publishers return 403 to scripts), so it should not block merges.

Suggested approach: a script that collects external URLs from the markdown files, requests each with a browser-like user agent and a timeout, treats 2xx and 3xx as fine, 403 and 429 as unknown (reported separately, never failing), and 404 or a DNS failure as broken. Run it weekly from a scheduled workflow that opens or updates one issue listing the broken links instead of failing a build. Add a --limit option and a cache so a run stays under a few minutes.

Done when: the script has tests with a local test server for each status class, the weekly workflow runs with a timeout and a concurrency group, and it reports (never fails the repository's checks).