Add backend scheduling

[ssavutu]

Summary

Adds backend-owned scheduling for articles and polls.

• Adds scheduled_pub_date for articles and promotes due scheduled articles into pub_date from a server-side scheduler.
• Reconciles due scheduled polls so the previously live poll is closed when a queued poll takes over.
• Keeps public article surfaces, comments, search, sitemap, SEO audit, and taxonomy counts pinned to actually live articles.
• Updates the CMS article editor/list UI to schedule articles and display scheduled status.

Validation

• go test ./...
• npm run build

Note: the frontend build still reports the existing large chunk warning.