docs(cropper): explain why the queue's core label maps to uc

Evidence: GET {a2loft}/identity/{code}/units resolves domain codes — uc,
ucth, mtr, uri, yrka are valid, core answers 'Invalid domain code
specified.' The label comes from the Telegram intake hashtag, not a2loft.

tools/cropper.js

@@ -12,7 +12,17 @@ import { stripPhonePrefix, cropPermission } from './permit.js';
 const require = createRequire(import.meta.url);
 
 const QUEUE_NAME = 'id-images';
-/** Queue domains that differ from their a2loft code (legacy bot behaviour). */
+/**
+ * Queue domains that differ from their a2loft identity code.
+ *
+ * A queue item's `domain` comes from the Telegram intake: the operator tags a
+ * photo (e.g. `#core`) and the bot stores that label verbatim, then translated
+ * it before every a2loft call. a2loft has no `core` domain — `GET
+ * {a2loft}/identity/core/units` answers "Invalid domain code specified." while
+ * `uc` is valid — so the label must be mapped before the save is forwarded.
+ * (The proper fix is on the intake side: enqueue the a2loft code, or register
+ * `core` upstream; until then this keeps existing queue data working.)
+ */
 const DOMAIN_ALIASES = { core: 'uc' };
 const BUCKET = 'files';
 const REMOVEBG_URL = 'https://removebg.itcass.net/api/removebg';