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/libraries/[id]/series/[name]/page.tsx

@@ -94,7 +94,7 @@ export default async function SeriesDetailPage({
                 alt={t("books.coverOf", { name: displayName })}
                 fill
                 className="object-cover"
-                unoptimized
+                sizes="160px"
               />
             </div>
           </div>