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/Models/MailwizzConfig.php

@@ -21,6 +21,7 @@ class MailwizzConfig extends Model
         'field_first_name',
         'field_last_name',
         'field_phone',
+        'field_coupon_code',
         'tag_field',
         'tag_value',
     ];