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>

test/admin-role.test.js

@@ -21,7 +21,8 @@ test('only admins can promote or demote, nobody can demote themselves, and chang
     const port = await freePort(), output = [];
     child = spawn(process.execPath, ['server.js'], { cwd: path.resolve(__dirname, '..'), windowsHide: true,
       env: { ...process.env, LIBRARY: library, DATA: data, PORT: String(port), SCAN_INTERVAL: '0', FULL_SCAN_INTERVAL: '0',
-        SCRAPER_URL: 'http://127.0.0.1:9', SCRAPER_TOKEN: 'fixture-token', SCRAPER_TOKEN_FILE: '', BACKUP: '' } });
+        SCRAPER_URL: 'http://127.0.0.1:9', SCRAPER_TOKEN: 'fixture-token', SCRAPER_TOKEN_FILE: '', BACKUP: '',
+        GREATEST_COVER_WARM: '0' } });
     child.stdout.on('data', d => output.push(String(d)));
     child.stderr.on('data', d => output.push(String(d)));
     const base = `http://127.0.0.1:${port}`;