Add site-wide AI-powered content insights

[jeffpaul]

Explore AI-powered insights that analyze content across the site, such as identifying themes, gaps, trends, or opportunities. This is distinct from editor-level assistance and focuses on site-wide analysis.

This feature should be experimental and avoid automated changes.

Acceptance criteria:

• AI analysis operates across multiple posts or content types
• Insights are informational and non-destructive
• Results are presented clearly and accessibly in the admin
• Performance and cost considerations are documented