feat: add postcode resolution to /api/stations and Filament SearchResource

Extends NearbyStationsRequest to accept `postcode` (full or outcode) as an alternative to lat/lng. PostcodeService resolves it via postcodes.io and falls through to coordinates. Also adds SearchResource to the Filament admin panel for viewing logged search activity with fuel type filter and price/distance stats columns. Includes SQLite GREATEST/LEAST function polyfills in AppServiceProvider for test compatibility.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Http/Requests/Api/NearbyStationsRequest.php

@@ -15,8 +15,9 @@ class NearbyStationsRequest extends FormRequest
     public function rules(): array
     {
         return [
-            'lat' => ['required', 'numeric', 'between:-90,90'],
-            'lng' => ['required', 'numeric', 'between:-180,180'],
+            'postcode' => ['nullable', 'string', 'max:10'],
+            'lat' => ['required_without:postcode', 'nullable', 'numeric', 'between:-90,90'],
+            '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'],