Named application roots and external URLs with AppUrlOptions, resolved tenant-aware via IAppUrlProvider. USE FOR: AppUrlOptions.Applications; RootUrl and named Urls; email links; cross-application links; tenantId and tenantName placeholders; redirect allow-list checks. DO NOT USE FOR: rendering the email body (render-text-templates) or sending it (send-emails); remote service base addresses (consume-remote-services); resolving tenants by domain (configure-multi-tenancy).
