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>

tools/test-compose.js

@@ -69,6 +69,7 @@ for (const [index, name] of ['Moon Garden', 'Before the Rain', 'Iron Harbor'].en
 const env = { ...process.env, LIBRARY_PATH: library, DATA_PATH: data, SCRAPER_DATA_PATH: path.join(data, 'scraper'),
   SCRAPER_TOKEN_PATH: tokenFile, BACKUP_PATH: backupDir, READER_PORT: port, READER_BIND: '127.0.0.1', COOKIE_SECURE: '0', APP_IMAGE: process.env.APP_IMAGE || 'mangacircus:local',
   READER_IMAGE: process.env.APP_IMAGE || 'mangacircus:local',
+  GREATEST_COVER_WARM: '0', // QA stacks never download catalog covers
   ...(readingGateQa ? { READING_WINDOWS_JSON: '{"0":[],"1":[],"2":[],"3":[],"4":[],"5":[],"6":[]}',
     READING_TIMEZONE: 'Europe/Berlin', READING_QUIZ_UNLOCK_MINUTES: '60', READING_QUIZ_CATALOG_PATH: '/data/reading-quiz.json' }
     : { READING_WINDOWS_JSON: '', READING_QUIZ_CATALOG_PATH: '' }),