Sitemap URLs no longer contain "undefined"

A handful of URLs in the production sitemap were being generated with the literal token undefined in the path — things like https://undefined/foo/bar. The root cause was a runtime config key not being available at sitemap generation time, so the URL builder coerced undefined into the host position.

The config is now resolved upfront before sitemap generation runs, so every URL that lands in the sitemap is a real, fully-qualified URL. Search engines can fetch every entry without falling into a black hole.