Greatest: serve catalog covers from a local copy (BL-0053)

The page hotlinked ~100 AniList CDN covers per load and some failed at random.
The reader now downloads each catalog cover once (sequential, ~1/s, stops on
401/403/429, images up to 2 MB) into data/greatest-covers/ and serves it from
/api/greatest-cover with a year-long browser cache; until a copy exists the
route redirects to the catalog URL. The page also accepts the server's
relative cover URLs, so owned titles show their library cover again.
GREATEST_COVER_WARM=0 turns the downloads off (tests, offline installs).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/greatest.js

@@ -65,8 +65,10 @@
 
   function cover(item) {
     const box = h('span', { class: 'greatest-cover' }, h('span', { class: 'greatest-cover-fallback', 'aria-hidden': 'true' }, item.title.slice(0, 1)));
-    if (item.cover && safeHref(item.cover)) {
-      const image = h('img', { src: item.cover, alt: '', loading: 'lazy' });
+    // Covers come from the server (cached catalog covers, owned library covers) as relative URLs.
+    const src = /^(api\/greatest-cover\?|lib\/)/.test(item.cover || '') ? item.cover : safeHref(item.cover);
+    if (src) {
+      const image = h('img', { src, alt: '', loading: 'lazy' });
       image.onerror = () => image.remove();
       box.append(image);
     }