feat: offer a newsletter opt-in for the other participants

addresses #869etn6c5

src/Enum/NewsletterOptInContext.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace App\Enum;
+
+/**
+ * Who asked for a double opt-in mail — the recipient themselves, or the applicant of a
+ * booking they are travelling on. Only the wording of the confirmation mail depends on
+ * this: a third-party sign-up has to name who requested it and make clear that nothing
+ * is subscribed without the recipient's own click. Confirmation itself stays
+ * context-agnostic, so the value never has to be persisted.
+ */
+enum NewsletterOptInContext: string
+{
+    case Own = 'own';
+    case Companion = 'companion';
+
+    public function subject(): string
+    {
+        return match ($this) {
+            self::Own => 'Deine Anmeldung zum E&P Newsletter',
+            self::Companion => 'Möchtest du den E&P Newsletter erhalten?',
+        };
+    }
+
+    public function template(): string
+    {
+        return match ($this) {
+            self::Own => 'email/newsletter_opt_in.html.twig',
+            self::Companion => 'email/newsletter_opt_in_companion.html.twig',
+        };
+    }
+}