fix(backup): say what the backups console actually does

Two places claimed a stored payload cannot leave the page: the card's own
description ("payloads are not downloadable from this page") and the listing
route's comment ("admins can trigger a re-snapshot but not download another
user's payload from the UI"). Both have been false since the download button
shipped — every row carries one, and the route hands back the decrypted JSON.

The description now says what happens instead: sizes are the encrypted blob,
Download decrypts the snapshot for the admin who asks, the ciphertext stays in
the database. Seven catalogues, and the route comment points at the route that
owns the payload.

src/app/api/admin/backups/route.ts

@@ -2,9 +2,11 @@
  * GET /api/admin/backups — admin-only listing of `DataBackup` rows.
  *
  * Returns one row per (userId, type) pair. The `data` payload is NOT
- * shipped — only metadata (id, userId, username, type, size in bytes,
- * createdAt). The encrypted blob remains server-side; admins can trigger
- * a re-snapshot but not download another user's payload from the UI.
+ * shipped by THIS route — only metadata (id, userId, username, type, size in
+ * bytes, createdAt), so the listing costs a count and not a decrypt. The
+ * payload has its own route: `/api/admin/backups/<id>/download` decrypts one
+ * snapshot for the admin who asks for it, and the ciphertext stays in the
+ * database either way.
  *
  * Alongside the rows, the response says whether the SCHEDULE is still alive:
  * how old the newest scheduled copy is and how the last scheduled run ended.