Create a workout plan (async generation)

Schedules an asynchronous generation job and returns immediately with `202 Accepted` plus a `job_id`. Generation builds the workouts under the plan from the supplied preferences, constraints, and coach-derived member insights. Creating a new plan marks the previous active plan inactive — only one plan is active at a time. Poll `GET /v1/users/{user_id}/jobs/{job_id}` for completion or consume the partner queue update event.

curl -X POST "https://loading/v1/users/497f6eca-6276-4993-bfeb-53cbbbba6f08/workout-plans" \  -H "Content-Type: application/json" \  -d '{    "title": "string"  }'

{
  "data": {
    "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",
    "job_id": "453bd7d7-5355-4d6d-a38e-d9e7eb218c3f",
    "status": "accepted"
  }
}

{
  "detail": [
    {
      "loc": [
        "string"
      ],
      "msg": "string",
      "type": "string"
    }
  ]
}