POST /v2/legal-entities/:id/custodian-authorities

Create a custodian authority under a legal entity. The schema is
custodian-specific — see the public docs at
https://docs.aleta.io/api/custodian-integration/custodian-authorities
and the `custodian` and `custodianAuthFlow` relationships.

Path params:
  id (required) — Legal-entity identifier.

Request: schema depends on the chosen custodian-auth-flow. See
`aleta api --spec /v2/custodian-auth-flows/:id` for the per-flow shape.

Response (201 Created):
{
  "data": {
    "id": "string (UUID)",
    "type": "custodian-authority",
    "attributes": { /* see custodian-authority data model */ },
    "relationships": {
      "legalEntity": { "data": { "type": "legal-entity", "id": "string" } },
      "custodian":   { "data": { "type": "custodian", "id": "string" } }
    }
  }
}