LLM Backtesting and workbench

[minaattari]

This commit adds a manufacturing-stress forecasting workbench that:

Synchronizes project dependencies with uv sync.
Loads and refreshes FRED data.
Runs historical-frequency, logistic-regression, and XGBoost predictors.
Supports optional LLM backtesting using RUN_LLMP_BACKTEST.
Caches LLM results to avoid repeated token usage.
Uses the same Brier-score evaluation across all predictors.
Supports configurable backtest stride and current forecasts.
Adds the cached LLMP backtest runner, tests, and documentation.