subscribe : fix date saving, duplicate detection and display format

Fix inscription date key to lowercase for Kirby compatibility, fix
duplicate email check using array_column, change date display to
DD/MM/YYYY, and sort subscribers by inscription date.

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

site/blueprints/pages/newsletter.yml

@@ -12,15 +12,14 @@ tabs:
           subscribers:
             label: Abonnés
             type: structure
-            sortBy: email asc
+            sortBy: inscriptionDate desc
             fields:
               email:
                 type: email
               inscriptionDate:
                 label: Date d'inscription
                 type: date
-                display: MM/DD/YYYY
-                default: today
+                display: DD/MM/YYYY
       - width: 1/3
         sections:
           newsletters:

site/config/routes/subscribe.php

@@ -13,7 +13,7 @@ return [
           $page = page('lettre');
           $subscribers = $page->subscribers()->yaml();
 
-          $emailExists = in_array(['email' => $email], $subscribers);
+          $emailExists = in_array($email, array_column($subscribers, 'email'));
 
           if ($emailExists) {
               return [
@@ -24,7 +24,7 @@ return [
 
           $newSubscriber = [
             'email' => $email,
-            'inscriptionDate' => date('Y-m-d')
+            'inscriptiondate' => date('Y-m-d')
             ];
           $subscribers[] = $newSubscriber;