ANU-5: Switch to Local Postgres

This change replaces the CMS development database hosted by Supabase with a Docker Compose-managed local stack containing the CMS web application, PostgreSQL with pgvector, and pgAdmin. It also defines a guarded one-time export, restore, verification, and rollback process for the existing application data.

Change artifacts

• Change brief
• Repository impact
• Implementation issues
• Progress
• Completion review

Current state

• Linear parent: ANU-5
• Priority: High
• Status: Product completion review approved; completion PR pending
• Implementation issues: ANU-6 and ANU-7 are Done; both implementation PRs are merged
• Canonical publication: not published