chore(content): rename broken featured_image to legacy_featured_image

The WordPress-era `featured_image` paths (/wp-content/uploads/...) were
never migrated and 404 on both powershell.org and locally. Rename the
key to `legacy_featured_image` across all 191 posts to preserve the
values for potential recovery while freeing `featured_image` for real,
working images going forward. No template currently consumes the key,
so rendering is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: HeyItsGilbert