Retrieves an incident retrospective

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

{
  "data": {
    "id": "538795fa-5257-4e7d-82de-acd18df0d4bb",
    "type": "incident_post_mortems",
    "attributes": {
      "incident_id": "d2e993ff-9ed1-40e1-bec8-f8f2a09bcb52",
      "title": "Numquam asperiores quam ratione.",
      "status": "draft",
      "url": "https://test.rootly.com/account/post_mortems/numquam-asperiores-quam-ratione",
      "short_url": null,
      "content": null,
      "published_at": null,
      "started_at": "2025-12-23T23:19:16.239-08:00",
      "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": "2025-12-23T23:19:16.706-08:00",
      "updated_at": "2025-12-23T23:19:16.710-08:00"
    },
    "relationships": {
      "incident_retrospective_steps": {
        "data": [
          {
            "id": "fcef2efd-adb9-4e75-a180-920fc3e692e4",
            "type": "incident_retrospective_steps"
          }
        ]
      }
    }
  }
}