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/books/[id]/page.tsx

@@ -95,7 +95,7 @@ export default async function BookDetailPage({
               alt={t("bookDetail.coverOf", { title: book.title })}
               fill
               className="object-cover"
-              unoptimized
+              sizes="192px"
               loading="lazy"
             />
           </div>