feat: security hardening (Fasa 10)

- EnsureIsAdmin middleware: gates all admin routes on is_admin flag
- Apply admin middleware to entire admin route group
- Fix questionnaire resource route parameter name mismatch ({set})
- Audit log on questionnaire confirmation

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

app/Http/Middleware/EnsureIsAdmin.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace App\Http\Middleware;
+
+use Closure;
+use Illuminate\Http\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+class EnsureIsAdmin
+{
+    public function handle(Request $request, Closure $next): Response
+    {
+        if (! $request->user()?->is_admin) {
+            abort(403, 'Akses ditolak.');
+        }
+
+        return $next($request);
+    }
+}