Delete a finished or ended season run that published nothing.

A human decision: call only after the user explicitly agrees. Removes the run and its episode rows for good, so it leaves GET /story-projects/{projectId}/season-runs. What it made stays: its plans, jobs, finished videos and releases keep their own rows (they only lose the link back to the run), so videos stay in Videos and releases in Publishing. Refused with 409 season_run_state_invalid and one plain sentence when the run is still live (end it first with POST /season-runs/{runId}/end), when it published an episode, when one of its releases is scheduled or publishing, or while a recording, episode job or video render it started is still running. Free.