Retrieves an incident retrospective

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

{
  "data": {
    "id": "e96e0d02-ea22-423f-a958-ca4f70b5f6fa",
    "type": "incident_post_mortems",
    "attributes": {
      "incident_id": "2cfaae1b-34a8-41d3-904c-80f7ed118c7a",
      "title": "Ut quibusdam et possimus.",
      "status": "draft",
      "url": "https://test.rootly.com/account/post_mortems/ut-quibusdam-et-possimus",
      "short_url": null,
      "content": null,
      "published_at": null,
      "started_at": "2025-12-17T14:11:31.553-08:00",
      "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": "2025-12-17T14:11:31.936-08:00",
      "updated_at": "2025-12-17T14:11:31.941-08:00"
    },
    "relationships": {
      "incident_retrospective_steps": {
        "data": [
          {
            "id": "d3e47e4a-8086-42db-b6a4-74e6a98e4c6f",
            "type": "incident_retrospective_steps"
          }
        ]
      }
    }
  }
}