feat(covers): generate webp covers for posts missing cover art

Add scripts/generate-missing-covers.mjs to render OG-style cover cards as
WebP (quality-tuned, all under 30KB) for 42 posts without cover images.
Each post gets cover frontmatter, a .thumb.webp list thumbnail, and
updated posts.json via build.

Run: npm run generate:covers

Co-authored-by: 兰州小红鸡 <flyphp@outlook.com>

Author: cursoragent