Admin endpoint to set concurrency burst factor (#3412)

Example cURL call using an admin user PAT (replace with a real one):

```sh
curl -X PUT https://cloud.trigger.dev/admin/api/v1/environments/<environmentId>/burst-factor \
    -H "Authorization: Bearer tr_pat_1234" \
    -H "Content-Type: application/json" \
    -d '{"burstFactor": 1.5}'
```