feat: add GET /api/prediction endpoint

Implements PredictionRequest (fuel_type validation with ValueError→ValidationException), PredictionController delegating to NationalFuelPredictionService, and 5 feature tests. Also fixes LEAST() MySQL-only function to a CASE WHEN expression for SQLite test compatibility.

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

app/Http/Controllers/Api/PredictionController.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Http\Controllers\Api;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Api\PredictionRequest;
+use App\Services\NationalFuelPredictionService;
+use Illuminate\Http\JsonResponse;
+
+class PredictionController extends Controller
+{
+    public function __construct(
+        private readonly NationalFuelPredictionService $predictionService,
+    ) {}
+
+    public function index(PredictionRequest $request): JsonResponse
+    {
+        $fuelType = $request->fuelType();
+        $lat = $request->filled('lat') ? (float) $request->input('lat') : null;
+        $lng = $request->filled('lng') ? (float) $request->input('lng') : null;
+
+        $result = $this->predictionService->predict($fuelType, $lat, $lng);
+
+        return response()->json($result);
+    }
+}

app/Http/Requests/Api/PredictionRequest.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Http\Requests\Api;
+
+use App\Enums\FuelType;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\ValidationException;
+
+class PredictionRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'fuel_type' => ['required', 'string'],
+            'lat' => ['nullable', 'numeric', 'between:-90,90'],
+            'lng' => ['nullable', 'numeric', 'between:-180,180'],
+        ];
+    }
+
+    public function fuelType(): FuelType
+    {
+        try {
+            return FuelType::fromAlias($this->string('fuel_type')->toString());
+        } catch (\ValueError) {
+            throw ValidationException::withMessages(['fuel_type' => 'Unknown fuel type. Use: diesel, petrol, e10, e5, hvo, b10.']);
+        }
+    }
+}

app/Services/NationalFuelPredictionService.php

@@ -344,7 +344,7 @@ class NationalFuelPredictionService
             ->join('stations', 'station_prices.station_id', '=', 'stations.node_id')
             ->where('station_prices.fuel_type', $fuelType->value)
             ->where('station_prices.price_effective_at', '>=', now()->subDays(14))
-            ->whereRaw('(6371 * acos(LEAST(1.0, cos(radians(?)) * cos(radians(lat)) * cos(radians(lng) - radians(?)) + sin(radians(?)) * sin(radians(lat))))) <= 50', [$lat, $lng, $lat])
+            ->whereRaw('(6371 * acos(CASE WHEN (cos(radians(?)) * cos(radians(lat)) * cos(radians(lng) - radians(?)) + sin(radians(?)) * sin(radians(lat))) > 1.0 THEN 1.0 ELSE (cos(radians(?)) * cos(radians(lat)) * cos(radians(lng) - radians(?)) + sin(radians(?)) * sin(radians(lat))) END)) <= 50', [$lat, $lng, $lat, $lat, $lng, $lat])
             ->selectRaw('DATE(station_prices.price_effective_at) as day, AVG(station_prices.price_pence) as avg_price')
             ->groupBy('day')
             ->orderBy('day')