Bound the prerendered archive window

Prerendering every past answer page in six languages meant 700 static pages,
and the CI runner blew past Next's 60s per-page export timeout on
/puzzle/236, failing the build. The set also grew by six pages a day, so
every build would have been slower than the last.

generateStaticParams now covers the most recent 30 days per locale (202
pages, ~35s locally). Older days keep working exactly as before: they are
still linked, still in the sitemap and still indexable, but render on first
request and are then cached by ISR — measured at 180ms cold.

Also raises staticPageGenerationTimeout to 180s for headroom, since the
runner shares a host with Gitea and Postgres.

Refs #6

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QvQ1ErZNRUcWgCEkJ9uyrn

next.config.js

@@ -2,6 +2,9 @@
 const nextConfig = {
   htmlLimitedBots: /.*/,
   output: "standalone",
+  // The CI runner shares a host with other services, so a cold page render can
+  // occasionally crawl. 60s (the default) was not enough headroom.
+  staticPageGenerationTimeout: 180,
 };
 
 module.exports = nextConfig;