PERF.11 Website: /sitemap.xml takes 3.1 s and 6,042 SQL queries on every crawl

Measured 2026-11-08 (bin/admin-perf.php /sitemap): 3,118 ms, 6,042 queries (2,080 ms SQL): LibraryItems::browse x40 (one page of 100 per call, 1,293 ms: COUNT + LEFT JOIN library_versions + ORDER over 13,850 items each time), then per item LibraryVersions::byId, Auth::byId and LibraryAuthors::uuidFor (2,000 each). Crawlers fetch the sitemap all day; it is 474 KB of XML.

Plan: one query for the library URLs (item slug/uuid + author uuid + current version date, joined, no per-row lookups), written to a cached file (storage/cache/sitemap.xml, rebuilt by cron hourly or on library publish) and served from there; keep the dynamic parts (news, tutorials, apps) as they are. Target under 50 ms, under 10 queries.