Switch Instagram embeds to iframe-only approach

Replace InstagramPlugin (which loads external embed.js script) with
InstagramEmbedder that generates iframes directly.

Changes:
- Add InstagramEmbedder following YouTube/Vimeo/3speak pattern
- Register InstagramEmbedder in AssetEmbedder
- Remove InstagramPlugin from blog renderer plugins
- Add tests for InstagramEmbedder

Benefits:
- No external scripts loaded (privacy improvement)
- Consistent with other video/media embedders
- Simpler CSP: only frame-src needed, not script-src
- Fixes eager script loading issue for Instagram

Relates to #786