perf: enable Next.js image optimization across backoffice

Remove `unoptimized` flag from all thumbnail/cover Image components
and add proper responsive `sizes` props. Convert raw `<img>` tags on
the libraries page to next/image. Add 24h minimumCacheTTL for
optimized images. BookPreview keeps `unoptimized` since the API
already returns optimized WebP.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/app/components/BookCard.tsx

@@ -51,7 +51,6 @@ function BookImage({ src, alt }: { src: string; alt: string }) {
         sizes="(max-width: 640px) 50vw, (max-width: 768px) 33vw, (max-width: 1024px) 25vw, 16vw"
         onLoad={() => setIsLoaded(true)}
         onError={() => setHasError(true)}
-        unoptimized
       />
     </div>
   );