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

Upload a proof-of-access file for a custodian authority. Unlike most
endpoints, this is a multipart/form-data upload — NOT a JSON:API
request.

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

Request:
  Content-Type: multipart/form-data
  Form fields:
    name (required, string)  — Logical role of the file. Must match
                               one of `expectedFiles` on the auth flow
                               (e.g. "power-of-attorney", "consent").
    file (required, binary)  — The file contents.

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "custodian-authority-file",
    "attributes": {
      "hash":       "string — algorithm-prefixed",
      "name":       "string",
      "uploadedAt": "ISO 8601 timestamp",
      "filename":   "string"
    }
  }
}

Note: the CLI's `aleta api` command currently sends a JSON body — it
doesn't yet handle multipart uploads. Use a different tool (curl) to
exercise this endpoint until multipart support is added.