feat: admin editable email templates

addresses #869eg7ptr

src/Model/RenderedEmailTextDto.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Model;
+
+/**
+ * The finished, already escaped parts of an editable mail.
+ *
+ * Every string in here is HTML-safe by construction (see App\Email\EmailTextRenderer),
+ * which is what lets email/generic.html.twig print them with |raw. Do not build one of
+ * these anywhere but in the renderer.
+ */
+readonly class RenderedEmailTextDto
+{
+    public function __construct(
+        public string $subject,
+        public string $headline,
+        public string $bodyHtml,
+    ) {
+    }
+}