feat: add Weeztix OAuth, coupon codes, and Mailwizz mapping

Implement Weeztix integration per documentation: database config and
subscriber coupon_code, OAuth redirect/callback, admin setup UI with
company/coupon selection via AJAX, synchronous coupon creation on public
subscribe with duplicate and rate-limit handling, Mailwizz field mapping
for coupon codes, subscriber table and CSV export, and connection hint
on the pages list.

Made-with: Cursor

app/Http/Controllers/Admin/PageController.php

@@ -28,6 +28,7 @@ class PageController extends Controller
     {
         $query = PreregistrationPage::query()
             ->withCount('subscribers')
+            ->with('weeztixConfig')
             ->orderByDesc('start_date');
 
         if (! $request->user()?->isSuperadmin()) {