POST /v2/transactions — cash-transfer-transaction

A cash movement on an account. Use the `counterparty-transaction`
relationship to link two cash-transfer transactions for an internal
transfer between two accounts; omit it for an external transfer
(e.g. deposit / withdrawal to/from a bank account outside Aleta).

Note: When deleting a cash-transfer transaction that has a counterparty,
DELETE /v2/transactions/:id removes only the specified transaction;
the counterparty is updated to drop the link.

Request:
{
  "data": {
    "type": "cash-transfer-transaction",
    "attributes": {
      "accountAmount": "number (required) — amount deposited/withdrawn, in account currency",
      "comment":       "string (optional)",
      "tradeDate":     "string, ISO 8601 (required) — trade date, end-of-day",
      "valueDate":     "string, ISO 8601 (required) — value date, end-of-day"
    },
    "relationships": {
      "account":                 { "data": { "type": "account", "id": "string (required)" } },
      "counterparty-transaction": {
        "data": { "type": "cash-transfer-transaction", "id": "string (optional)" }
      }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "cash-transfer-transaction",
    "attributes": { /* same as request */ },
    "relationships": { /* same as request */ }
  }
}