Add OG Image field to content editor SEO panel

[jdevalk]

Problem

The _emdash_seo table has a seo_image column and the content API accepts seo.image on create/update, but the admin UI's SEO panel doesn't expose an image field. The only fields shown are SEO Title, Meta Description, Canonical URL, and noIndex toggle.

This means there's no way to set an OG image per post through the admin interface — only via the API.

Proposed solution

Add an OG Image picker (using the existing MediaPickerModal component) to the content editor. Specifically:

1. Add a SeoImageField component that reuses MediaPickerModal to select an image from the media library and stores the URL in seo.image
2. Place it next to the Featured Image field in a 2-column grid layout, so both images are visible side by side when editing
3. Add description support to ImageFieldRenderer so both image fields can show helper text below the image:
   • Featured Image: "Used as the main visual for this post on listing pages and at the top of the post"
   • OG Image: "Image shown when this page is shared on social media"

Notes

• The database and API already fully support seo_image — this is purely an admin UI gap
• The SeoPanel comment even mentions "OG image" as an intended field but it was never implemented
• PR: Add OG Image field to content editor #328