feat: admin editable email templates

addresses #869eg7ptr

src/Validator/KnownPlaceholders.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Validator;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Rejects {tokens} the mail being edited does not know.
+ *
+ * Without this a typo like {destinaton} would reach a teamer's inbox as a literal token
+ * or, worse, silently as a dash.
+ */
+#[\Attribute(\Attribute::TARGET_PROPERTY | \Attribute::IS_REPEATABLE)]
+class KnownPlaceholders extends Constraint
+{
+    public string $message = 'Unbekannter Platzhalter: {{ placeholder }}. Erlaubt sind: {{ allowed }}';
+
+    /**
+     * @var string[]
+     */
+    public array $allowed = [];
+
+    /**
+     * @param string[] $allowed
+     */
+    public function __construct(array $allowed = [], ?string $message = null, ?array $groups = null, mixed $payload = null)
+    {
+        parent::__construct([], $groups, $payload);
+
+        $this->allowed = $allowed;
+        $this->message = $message ?? $this->message;
+    }
+}