refactor: extract HaversineQuery helper, fix LL bind quirk

The 5 haversine SQL fragments duplicated across StationController and
NationalFuelPredictionService disagreed on float-clamping (LEAST only,
GREATEST/LEAST, vs. CASE WHEN). Centralised in App\Services\HaversineQuery
with the safe GREATEST(-1.0, LEAST(1.0, …)) form everywhere.

withinKm() embeds the radius as a numeric literal (sprintf %F) because
PDO + SQLite binds float parameters as strings by default, which breaks
numeric comparison against the haversine expression — a NULL filter would
silently match all rows. Coordinates remain bound positionally.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Http/Controllers/Api/StationController.php

@@ -9,6 +9,7 @@ use App\Http\Resources\Api\StationResource;
 use App\Models\Search;
 use App\Models\Station;
 use App\Models\User;
+use App\Services\HaversineQuery;
 use App\Services\NationalFuelPredictionService;
 use App\Services\PlanFeatures;
 use App\Services\PostcodeService;
@@ -43,14 +44,12 @@ class StationController extends Controller
         $radius = $request->radius();
         $sort = $request->sort();
 
+        [$distanceSql, $distanceBindings] = HaversineQuery::distanceKm($lat, $lng);
+
         $all = Station::query()
             ->selectRaw(
-                'stations.*, spc.price_pence, spc.fuel_type, spc.price_effective_at,
-                (6371 * acos(GREATEST(-1.0, LEAST(1.0,
-                    cos(radians(?)) * cos(radians(lat)) * cos(radians(lng) - radians(?))
-                    + sin(radians(?)) * sin(radians(lat))
-                )))) AS distance_km',
-                [$lat, $lng, $lat],
+                "stations.*, spc.price_pence, spc.fuel_type, spc.price_effective_at, {$distanceSql} AS distance_km",
+                $distanceBindings,
             )
             ->join('station_prices_current as spc', function (JoinClause $join) use ($fuelType): void {
                 $join->on('stations.node_id', '=', 'spc.station_id')