Add sitemap endpoints for SEO crawling

[raymondjacobson]

Summary

• Ports sitemap functionality from the discovery provider to serve XML sitemaps at the same paths (/sitemaps/default.xml, /sitemaps/{type}/index.xml, /sitemaps/{type}/{n}.xml)
• Root sitemap index (default.xml) references defaults.xml (static routes), plus track/playlist/user type indexes so crawlers can discover everything from a single entry point
• Filters match the original: only includes entities from users with >=10 followers, excludes unlisted/deleted/private content
• Count queries use stale-while-revalidate caching (1hr TTL) — returns stale value immediately and refreshes in the background to avoid slow responses from the 4-table JOIN counts

Test plan

• TestSitemapDefault — root index references all sub-sitemaps
• TestSitemapDefaults — static routes urlset contains expected pages
• TestSitemapTrackIndex — track index generates page links
• TestSitemapTrackPage — includes listed tracks, excludes unlisted/deleted/low-follower
• TestSitemapPlaylistPage — includes public playlists/albums, excludes private/low-follower, uses correct album/playlist prefix
• TestSitemapUserPage — includes eligible users, excludes deactivated/low-follower
• TestSitemapInvalidType — returns 400 for unknown type
• TestSitemapInvalidPage — returns 400 for non-numeric page
• TestSitemapContentType — response is valid XML

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