feat: add price classification enum and reliable sort option

app/Http/Resources/Api/StationResource.php

@@ -2,6 +2,7 @@
 
 namespace App\Http\Resources\Api;
 
+use App\Enums\PriceClassification;
 use Illuminate\Http\Request;
 use Illuminate\Http\Resources\Json\JsonResource;
 use Illuminate\Support\Carbon;
@@ -26,6 +27,12 @@ class StationResource extends JsonResource
             'price_updated_at' => $this->price_effective_at
                 ? Carbon::parse($this->price_effective_at)->toISOString()
                 : null,
+            'price_classification' => PriceClassification::fromUpdatedAt(
+                $this->price_effective_at ? Carbon::parse($this->price_effective_at) : null
+            )->value,
+            'price_classification_label' => PriceClassification::fromUpdatedAt(
+                $this->price_effective_at ? Carbon::parse($this->price_effective_at) : null
+            )->label(),
         ];
     }
 }