feat: add hero image processing and API for serving variants

- Introduced a new script `prepare-hero-variants.mjs` to generate responsive hero image variants in WebP format.
- Added a fallback image `hero_fallback.png` for each variant.
- Created an API endpoint `hero-images.get.js` to retrieve available hero image variants and their fallback images.
- Implemented directory and file checks to ensure the existence of required images before serving.

package.json

@@ -19,6 +19,7 @@
     "check-security": "node scripts/verify-no-public-writes.js",
     "smoke-local": "BASE_URL=http://127.0.0.1:3100 node scripts/smoke-tests.js",
     "sync-public-data": "node scripts/sync-public-data.js",
+    "hero:prepare": "node scripts/prepare-hero-variants.mjs",
     "import-spielplan": "node scripts/import-spielplan.js",
     "publish-spielplan": "node scripts/publish-imported-spielplan.js",
     "playstore:assets": "./scripts/playstore-assets.sh",