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/Models/User.php

@@ -4,6 +4,8 @@ namespace App\Models;
 
 // use Illuminate\Contracts\Auth\MustVerifyEmail;
 use Database\Factories\UserFactory;
+use Filament\Models\Contracts\FilamentUser;
+use Filament\Panel;
 use Illuminate\Database\Eloquent\Attributes\Fillable;
 use Illuminate\Database\Eloquent\Attributes\Hidden;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
@@ -14,7 +16,7 @@ use Laravel\Fortify\TwoFactorAuthenticatable;
 
 #[Fillable(['name', 'email', 'password'])]
 #[Hidden(['password', 'two_factor_secret', 'two_factor_recovery_codes', 'remember_token'])]
-class User extends Authenticatable
+class User extends Authenticatable implements FilamentUser
 {
     /** @use HasFactory<UserFactory> */
     use HasFactory, Notifiable, TwoFactorAuthenticatable;
@@ -32,6 +34,11 @@ class User extends Authenticatable
         ];
     }
 
+    public function canAccessPanel(Panel $panel): bool
+    {
+        return $this->is_admin === true;
+    }
+
     /**
      * Get the user's initials
      */