GET /v3/instruments/:id/interest-rates

List interest-rate observations for an instrument.

Path params:
  id (required) — Instrument identifier (Mongo ObjectID).

Response (200 OK):
{
  "data": [
    {
      "id": "string",
      "type": "interest-rate",
      "attributes": {
        "date":     "ISO 8601 date",
        "value":    "number — annual rate as a decimal (0.05 = 5%)",
        "provider": "string — 'client' or 'aleta'"
      }
    }
  ]
}