license preamble added

src/main.rs

@@ -1,3 +1,21 @@
+/*
+kosyncrs - A reading progress sync server for KOReader
+Copyright (C) 2025  Martin Brodbeck <martin@brodbeck-online.de>
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
 use axum::{
     extract::{Path, State},
     http::HeaderMap,