Performance V80

Changes in this build are performance-only and preserve the existing application behavior:
- Removed subscriber auto-bootstrap from every normal GET/page refresh.
- Subscriber, campaign, and comment jobs continue through background workers / scheduler.
- Moved runtime schema checks out of db() so normal requests do not repeatedly query INFORMATION_SCHEMA or run ALTER TABLE checks.
- Cached settings for the lifetime of a request.
- Background-table migrations are one-time and keyed in settings.
- Utility button-column migration is one-time and only checked on Utility Messages.
- Added one-time database indexes for common page/subscriber/template/campaign/job filters.
- Removed the duplicate unbounded Facebook-account query and N+1 subscriber/account queries from the connected-pages table.
- Reduced dashboard activity polling from 10s to 15s.
- Reduced the fire-and-forget worker connection timeout so starting a background job does not hold the browser request unnecessarily.

Background operation:
Keep the existing scheduler Cron configured. The site does not need to stay open for queued work to continue.
