feat: add user preferences and saved stations API endpoints

Adds authenticated endpoints for reading/updating fuel type preferences and managing saved stations, backed by new migrations and a SavedStation model.

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

app/Models/SavedStation.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+final class SavedStation extends Model
+{
+    protected $fillable = ['user_id', 'station_id'];
+}