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>

docs/deployment.md

@@ -21,7 +21,7 @@ Data on the host, all bind mounts (no named volumes):
 | Path (`.env`) | Contents | Back up? |
 |---|---|---|
 | `LIBRARY_PATH` | The manga: `Manga/Book/Chapter/images` ([library-format.md](library-format.md)) | Yes, it is your collection |
-| `DATA_PATH` | `db.json` (users, progress, votes), catalog, discovery feeds, library index cache | Yes (built-in backup, section 8). `discovery-cache/` inside it is safe to delete and may be left out of backups: the next harvest rebuilds it |
+| `DATA_PATH` | `db.json` (users, progress, votes), catalog, discovery feeds, library index cache | Yes (built-in backup, section 8). `discovery-cache/` inside it is safe to delete and may be left out of backups: the next harvest rebuilds it. The same holds for `greatest-covers/` (local copies of the Greatest catalog's cover images, fetched again after the next start) |
 | `SCRAPER_DATA_PATH` | Queue, settings, source ledger, metadata cache | Included in the built-in backup |
 | `BACKUP_PATH` | Built-in backup archives | Keep it on another disk or NAS |
 | `SCRAPER_TOKEN_PATH` | Shared secret between reader and scraper | Regenerate if lost |
@@ -71,6 +71,7 @@ demo manga in an empty library after the build in section 5:
 | `READING_WINDOWS_JSON`, `READING_TIMEZONE`, `READING_QUIZ_*` | off | Optional server-enforced reading hours ([reading-limiter.md](reading-limiter.md)) |
 | `DISCOVERY_HARVEST_AT` | `03:30` | Daily time (`HH:MM`) of the nightly Discover harvest from AniList, in the container's local time zone (`TZ`). Only users who opted in are harvested; the admin can change the time or switch it off on the Discover page ([discovery.md](discovery.md)) |
 | `DISCOVERY_ANILIST_URL` | AniList's public API | Tests only: points the discovery harvest at a fixture server. Leave unset in production |
+| `GREATEST_COVER_WARM` | `1` | `0` stops the reader from downloading the Greatest catalog's covers (about 100 images, one per second, 10 s after start and once a day for missing ones). Tests and offline installs set `0`; the page then loads covers from the catalog's own URLs |
 | `APP_IMAGE`, `VCS_REF` | `mangacircus:local`, `development` | Image to run; set a pinned registry image to deploy prebuilt releases (section 7) |
 | `COMPOSE_FILE` | `docker-compose.yml` | Extra Compose files, `:`-separated (e.g. `docker-compose.yml:deploy/compose.https.yml`) |
 | `MANGACIRCUS_DOMAIN` | – | Host name for `deploy/compose.https.yml` |