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>

docker-compose.yml

@@ -60,6 +60,7 @@ services:
       SCRAPER_TOKEN_FILE: /run/secrets/scraper_token
       DISCOVERY_HARVEST_AT: ${DISCOVERY_HARVEST_AT:-03:30}
       DISCOVERY_ANILIST_URL: ${DISCOVERY_ANILIST_URL:-}
+      GREATEST_COVER_WARM: ${GREATEST_COVER_WARM:-1}
     networks: [backend, egress]
     healthcheck:
       test: [CMD, node, tools/healthcheck.js, reader]