feat: add satori-based OG image generation pipeline

Generate 1200x630 OG images for every lesson and a default
landing page image using satori + @resvg/resvg-js. Dark
gradient background with lesson title, module name, and
branding. Runs as first postbuild step before sitemap and
prerender.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: beihaili