feat: move some forecasting fields to config #1969
Description
Context
We want to move some forecasting schema fields to the config so that altering those fields results in a different data source when creating new forecasts.
The motivation is that information about these fields is not exposed in the UI for forecasts. While some properties are visible—such as end_date (when forecasts end) and max_forecast_horizon (how far into the future we predict)—other critical pieces of information are not.
For example, we currently cannot see:
- when the training data starts (
start_date) - how much historical data was used (
train_period) - how often retraining happens (
retrain_frequency) - data-quality constraints that affect training
Because this information is not available in the forecast UI or beliefs, changing these fields silently changes the forecasts without any traceability.
To avoid this and to make forecasts reproducible and easier to reason about, these fields should live in the configuration, not in the runtime parameters.