Add a crawl schedule to a site

Add a named crawl schedule to a site: a crawl on cron (5 fields, UTC), optionally under its own max_pages, compared with the one before (auto_compare, on by default), with reports exported and the team emailed when it lands (email_on_complete, on by default). Your human sees it in the site's settings and can change or remove it there. A site whose billing is read-only cannot schedule (BILLING_REQUIRED). An agent may schedule at most 5 crawls a day for a site, every enabled schedule counted over the next 24 hours: a write past it is VALIDATION_FAILED; choose a slower schedule, or ask your human to set it in Frontshelf. Next: schedules_list to see when it runs next.