perf(assets): convert images to webp and check asset paths

Convert 26 PNG/JPG assets to WebP: max 1600 px, q80, diagrams lossless.
src/assets shrinks from 8.3 MB to 4.2 MB.
Use me.jpg for og:image and twitter:image; some crawlers skip WebP.
Add scripts/check-assets.mjs to lint, so a wrong asset path fails lint,
not only at runtime.

.gitattributes

@@ -46,6 +46,7 @@ Dockerfile  text eol=lf
 *.jpeg binary
 *.gif  binary
 *.ico  binary
+*.webp binary
 *.pdf  binary
 *.zip  binary
 *.tar  binary