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

resources/views/public/page.blade.php

@@ -71,6 +71,7 @@
                     'redirectUrl' => filled($redirectAfterSubmit) ? $redirectAfterSubmit : null,
                     'strings' => [
                         'linkCopied' => __('Link gekopieerd!'),
+                        'couponCopied' => __('Kortingscode gekopieerd!'),
                         'redirectCountdown' => __('You will be redirected in :seconds s…'),
                     ],
                 ]))"