Article updates now track every change with full history

This release introduces a proper article history system that tracks every change you make.

What's now stored:

• A new articleHistory table records each update as a discrete revision.
• Every revision is attributed to the user who made it via a new changed_by column on the articles table.
• Update operations are atomic — either the full update succeeds, or nothing changes — with rollback support if something fails partway through.
• Scheduled-article validation now correctly checks dates and time-zones before locking in a publish slot.

The store also handles updates optimistically, so the editor stays snappy even on large articles. You see your edit immediately; the network roundtrip happens in the background.

This is the foundation that the change-history viewer (also shipping around the same time) is built on. Together, they turn the article editor into a tool you can trust with serious work.