Add Cache-Control for immutable assets

curl -I .../_app/immutable/entry/start.*.js returns etag and content-encoding: gzip but no Cache-Control. Every JS chunk, CSS file, font and AVIF is content-hashed, so it can be cached for a year. Fonts (97 KB) and the largest chunks are re-validated on every page view today.

Tasks

• nginx: location /_app/immutable/ with add_header Cache-Control "public, max-age=31536000, immutable"
• HTML and __data.json: no-cache or a short max-age so deploys propagate
• Consider precompress: true in the adapter and gzip_static/brotli_static in nginx