Create/update the per-org template override (validates + increments version)

curl --request PUT \
  --url https://api-sandbox.featherhq.com/v1/notifications/types/{event_type}/template \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "title": "<string>",
  "body": "",
  "fields": [
    {
      "label": "<string>",
      "value": "<string>"
    }
  ],
  "link_text": "<string>",
  "link_url": "<string>"
}
'

{
  "event_type": "<string>",
  "channel_kind": "<string>",
  "source": "<string>",
  "slots": {
    "title": "<string>",
    "body": "",
    "fields": [
      {
        "label": "<string>",
        "value": "<string>"
      }
    ],
    "link_text": "<string>",
    "link_url": "<string>"
  },
  "version": 123,
  "default_slots": {
    "title": "<string>",
    "body": "",
    "fields": [
      {
        "label": "<string>",
        "value": "<string>"
      }
    ],
    "link_text": "<string>",
    "link_url": "<string>"
  },
  "variables": {}
}