feat: implement newsletter subscription feature with new forms and database structure

- Added new newsletter subscription functionality to the participation page.
- Created a new `NewsletterForm` component to handle user subscriptions.
- Introduced `NewsletterSubscriptions` collection to manage subscriptions in the database.
- Developed migration script to set up necessary database tables and fields for the newsletter feature.
- Refactored participation content to remove deprecated newsletter phase data and integrate new structure.
- Updated the `Participation` page to utilize the new newsletter form and handle different application phases.

src/migrations/index.ts

@@ -9,6 +9,7 @@ import * as migration_20260707_123500_preistraeger_gallery from './20260707_1235
 import * as migration_20260707_124500_site_settings_preistraeger_placeholder from './20260707_124500_site_settings_preistraeger_placeholder';
 import * as migration_20260707_130500_preistraeger_awards_jury_members from './20260707_130500_preistraeger_awards_jury_members';
 import * as migration_20260712_120000_partner_media_only from './20260712_120000_partner_media_only';
+import * as migration_20260713_133000_newsletter_form_and_subscriptions from './20260713_133000_newsletter_form_and_subscriptions';
 
 export const migrations = [
   {
@@ -66,4 +67,9 @@ export const migrations = [
     down: migration_20260712_120000_partner_media_only.down,
     name: '20260712_120000_partner_media_only',
   },
+  {
+    up: migration_20260713_133000_newsletter_form_and_subscriptions.up,
+    down: migration_20260713_133000_newsletter_form_and_subscriptions.down,
+    name: '20260713_133000_newsletter_form_and_subscriptions',
+  },
 ];