feat(collections): add sitemap provider for public collection pages
Registers a johncms.sitemap_provider that emits URLs for each active
collection root, every reachable active section (paths built by walking the
tree; sections behind an inactive ancestor are skipped) and every published
item (getVisibleForSitemap streams them with a cursor), with lastmod from
updated_at. Items under an inactive section are skipped. Item repository gains
getVisibleForSitemap; DB test covers active-only output and unreachable skips.
collection root, every reachable active section (paths built by walking the
tree; sections behind an inactive ancestor are skipped) and every published
item (getVisibleForSitemap streams them with a cursor), with lastmod from
updated_at. Items under an inactive section are skipped. Item repository gains
getVisibleForSitemap; DB test covers active-only output and unreachable skips.