POST /v2/custodian-authorities/:id/link

Link an additional account or depository to an active custodian
authority. The new linked entry starts at status `in-progress` and
moves to `active` once the custodian acknowledges.

Path params:
  id (required) — Custodian-authority identifier (UUID).

Request:
{
  "data": {
    "type": "custodian-authority-link",
    "attributes": {
      "depositoryNumber": "string (optional) — depository number at the custodian",
      "accountNumber":    "string (optional) — account number at the custodian"
    }
  }
}

Provide either `depositoryNumber` or `accountNumber` (one per request).

Response (204 No Content):
  Empty body.

The new entry will appear in `linkedDepositories` or `linkedAccounts`
on the next GET.