feat: add updated and brand sort options to /api/stations endpoint

Extends NearbyStationsRequest validation to accept `sort=updated` and `sort=brand`. Updates StationController to use match expression with dedicated closures: 'updated' sorts by price_effective_at descending (newest first), 'brand' sorts alphabetically by brand_name, with existing price and distance options.

app/Http/Requests/Api/NearbyStationsRequest.php

@@ -20,7 +20,7 @@ class NearbyStationsRequest extends FormRequest
             'lng' => ['required_without:postcode', 'nullable', 'numeric', 'between:-180,180'],
             'fuel_type' => ['required', 'string'],
             'radius' => ['nullable', 'numeric', 'between:0.1,50'],
-            'sort' => ['nullable', 'string', 'in:price,distance'],
+            'sort' => ['nullable', 'string', 'in:price,distance,updated,brand'],
             'pricing_mode' => ['nullable', 'string', 'in:pump'],
         ];
     }