POST /v2/limits

Create a new limit. The schema mirrors the resource page —
`limitFilter` and `limitType` are nested objects.

Request:
{
  "data": {
    "type": "limit",
    "attributes": {
      "name":        "string (required)",
      "ccyCode":     "string (required) — ISO 4217",
      "startDate":   "string, ISO 8601 (required)",
      "limitFilter": <see /v2/limits resource page>,
      "limitType":   <see /v2/limits resource page>
    },
    "relationships": {
      "client":      { "data": { "type": "client", "id": "string (required)" } },
      "parentLimit": { "data": { "type": "limit",  "id": "string (optional)" } }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string (UUID)",
    "type": "limit",
    "attributes": { /* same as request */ },
    "relationships": { /* same as request */ }
  }
}