feat: annotate license headers using reuse on text source files

templates/email/css/button.css

@@ -1,3 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Aravinth Manivannan <realaravinth@batsense.net>
+ *
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
 .button:hover {
   background-color: #993299;
   cursor: pointer;