Retrieves an incident retrospective

curl --request GET \
  --url https://api.rootly.com/v1/post_mortems/{id} \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "id": "64b64ab7-4562-4694-9e37-6c8cc025d48e",
    "type": "incident_post_mortems",
    "attributes": {
      "incident_id": "b9a7ed1a-3335-49e1-b6e3-44d204a8f78d",
      "title": "Veritatis enim aut dolores.",
      "status": "draft",
      "url": "https://test.rootly.com/account/post_mortems/veritatis-enim-aut-dolores",
      "short_url": null,
      "content": null,
      "published_at": null,
      "started_at": "2025-12-24T09:26:19.114-08:00",
      "mitigated_at": null,
      "resolved_at": null,
      "cancelled_at": null,
      "show_timeline": true,
      "show_timeline_starred_only": false,
      "show_timeline_genius": true,
      "show_timeline_trail": true,
      "show_timeline_tasks": true,
      "show_timeline_action_items": true,
      "show_timeline_order": "desc",
      "show_functionalities_impacted": true,
      "show_services_impacted": true,
      "show_groups_impacted": true,
      "show_action_items": true,
      "created_at": "2025-12-24T09:26:19.515-08:00",
      "updated_at": "2025-12-24T09:26:19.520-08:00"
    },
    "relationships": {
      "incident_retrospective_steps": {
        "data": [
          {
            "id": "6cc1f241-739b-4657-9cc0-39d7297e8e8a",
            "type": "incident_retrospective_steps"
          }
        ]
      }
    }
  }
}