/images/* is served cache-first from IMAGES_CACHE, which activate deliberately
keeps across builds. That is right for content whose URL is unique and wrong
for the app's own artwork: /images/icon-*, /images/logo-icon* and
/images/apple-splash/* keep their URLs while their contents change with the
logo, so a browser that had once loaded an icon would keep showing the old one
even after a deploy — and the apple-touch-icon is linked from index.html, so it
does travel through the page and the service worker.

Those paths now go network-first with a cached fallback (offline still works);
every other /images/ request is untouched.

Tests: frontend 148 passed; backend 1367 passed, 1 skipped.