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)

bootstrap/providers.php

@@ -1,9 +1,7 @@
 <?php
 
-use App\Providers\AppServiceProvider;
-use App\Providers\FortifyServiceProvider;
-
 return [
-    AppServiceProvider::class,
-    FortifyServiceProvider::class,
+    App\Providers\AppServiceProvider::class,
+    App\Providers\Filament\AdminPanelProvider::class,
+    App\Providers\FortifyServiceProvider::class,
 ];