refactor: split oil price ingestion and prediction into separate services + commands

- BrentPriceFetcher owns ingestion (fetchFromEia / fetchFromFred, each throws on failure)
- BrentPricePredictor owns prediction and marks latest brent_prices row as generated
- oil:fetch command tries EIA, falls back to FRED, fails loudly if both fail
- oil:predict command prompts if latest price already has a prediction; --force bypasses
- add prediction_generated_at column to brent_prices
- delete OilPriceService (replaced by the two focused services)

app/Services/BrentPriceSources/BrentPriceFetchException.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace App\Services\BrentPriceSources;
+
+use RuntimeException;
+
+final class BrentPriceFetchException extends RuntimeException {}