Download a forecast output file

Streams a single output file for a completed forecast job.

Available artifacts:

• forecast.json — point forecast values for each horizon month
• backtest_metrics.json — accuracy metrics from the backtest evaluation (only when backtest: true)
• backtest_trajectories.json — full trajectory samples from the backtest (only when backtest: true)
• external_signals.json — external driver signals used by the model
• input.json — the processed input timeseries as seen by the pipeline

The response body is the raw file bytes; the Content-Type header matches the artifact's content_type field from GET /api/v1/forecasts/{id}. Supports HTTP Range requests for partial downloads. Jobs tombstoned by the retention policy return 404.