feat: add Filament admin panel with migrations and design spec

- Add AdminPanelProvider mounting panel at `/admin` with `is_admin` auth guard
- Add `is_admin` boolean column to users table
- Add brent_prices and price_predictions tables with appropriate indexes
- Add comprehensive admin design spec covering resources, dashboard, navigation, and build order
- Configure default panel with amber primary color and standard middleware stack
- Add compiled Filament assets (actions.js, app.css)

app/Enums/PredictionSource.php

@@ -0,0 +1,9 @@
+<?php
+
+namespace App\Enums;
+
+enum PredictionSource: string
+{
+    case Llm = 'llm';
+    case Ewma = 'ewma';
+}