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/architecture.md

@@ -207,6 +207,7 @@ All requests and responses are JSON. POSTs require `Content-Type: application/js
 | `POST /api/admin/user/role` `{name, admin}` | admin | Grant or revoke admin rights; keeps the password and sessions. Revoking your own rights is refused, so one admin always remains |
 | `POST /api/admin/user/delete` `{name}` | admin | Delete a user and their data |
 | `GET /api/admin/backup` · `POST /api/admin/backup` `{command:"settings"\|"run"}` | admin | Built-in backup status, schedule/retention, "back up now" ([backup.md](backup.md)) |
+| `GET /api/greatest-cover?id=<entry>&v=<version>` | user | The Greatest entry's cover from `data/greatest-covers/` (`greatest/covers.js`), cached by the browser for a year; `v` changes with the catalog URL. A `302` to the catalog URL until the copy exists. `/api/greatest` lists this URL as `cover` for titles not in the library |
 | `GET /api/discovery` | user | The caller's discovery feed, settings, feedback and `refresh: { state: idle\|queued\|running, position?, nextAllowedAt?, blockedReason? }` (§6.6) |
 | `POST /api/discovery/feedback` `{candidateId, value}` | user | The caller's choice for a candidate; requires opt-in |
 | `POST /api/discovery/refresh` | user | Queue a per-user AniList harvest for the caller (opt-in required, else `403`). `{ harvestId, position }`; `409` while one is queued or running for the caller; `429 { nextAllowedAt }` within an hour of the last accepted refresh; `503 { blockedReason }` after AniList pushed back, until a nightly or admin harvest |