feat: auto-generate OpenGraph images for posts without photos

Uses Satori + @resvg/resvg-js to create branded 1200x630 social
preview cards at build time. Cards show post title, type badge,
date, and site name on a dark background with blue accent.

Generated images are cached in .cache/og/ (persistent on Cloudron)
and passthrough-copied to the output. Posts with photos continue
using their own images. Untitled posts (notes) use body text.

package.json

@@ -16,13 +16,17 @@
     "@11ty/eleventy-plugin-syntaxhighlight": "^5.0.2",
     "@atproto/api": "^0.12.0",
     "@chrisburnell/eleventy-cache-webmentions": "^2.2.7",
+    "@fontsource/inter": "^5.2.8",
     "@quasibit/eleventy-plugin-sitemap": "^2.2.0",
+    "@resvg/resvg-js": "^2.6.2",
     "eleventy-plugin-embed-everything": "^1.21.0",
+    "gray-matter": "^4.0.3",
     "html-minifier-terser": "^7.0.0",
     "markdown-it": "^14.0.0",
     "markdown-it-anchor": "^9.2.0",
     "pagefind": "^1.3.0",
-    "rss-parser": "^3.13.0"
+    "rss-parser": "^3.13.0",
+    "satori": "^0.19.2"
   },
   "devDependencies": {
     "@tailwindcss/typography": "^0.5.0",