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/PreregistrationPage.php

@@ -162,6 +162,11 @@ class PreregistrationPage extends Model
         return $this->hasOne(MailwizzConfig::class);
     }
 
+    public function weeztixConfig(): HasOne
+    {
+        return $this->hasOne(WeeztixConfig::class);
+    }
+
     public function isBeforeStart(): bool
     {
         return Carbon::now()->lt($this->start_date);