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/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.']);
+        }
+    }
+}