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);
+    }
+}