feat: page registry with its admin screen and signpost type

- golem15_seo_pages (migration 202610280002): the site's own frontend pages
  with an address and a name per locale, kind, origin, parent, content date
  and note; settings routes_reported_at
- SEO > Pages (golem15.seo.pages) after URL rules: list with kind, origin
  and missing filters and the missing row state, form with the P9 partial;
  the plugin's form extension saves the address and name per locale and
  refuses url_required, url_invalid, url_taken and url_owned
- menu item type seo-page with a content descriptor, so each page gets
  the SEO tab, resolves ok and is listed in the sitemaps grouped by kind
  (pages, offers, services, landing-pages, titled per locale)
- management API GET/POST /_seo/api/v1/pages, PUT/DELETE /pages/{id} and
  the MCP tools seo_list_pages, seo_create_page, seo_update_page,
  seo_delete_page (in this commit because the operations parity test maps
  every Pages operation to its tool)

controllers/admin_registry.go

@@ -32,6 +32,7 @@ func AdminControllers(app func() *backpack.App) []pact.AdminController {
 		redirectsAdminController{app: app},
 		notFoundAdminController{app: app},
 		rulesAdminController{app: app},
+		pagesAdminController{app: app},
 		robotsAdminController{app: app},
 		historyAdminController{app: app},
 		transferAdminController{app: app},