Add changelog entry for systemd service refresh

Author: nblock

CHANGELOG.md

@@ -31,6 +31,7 @@ HTTP API directly.
 ### Changes
 
 - Expiring or deleting a non-existent pre-auth key now returns an error instead of silently succeeding [#3324](https://github.com/juanfont/headscale/pull/3324)
+- Improve systemd service file hardening [#3341](https://github.com/juanfont/headscale/pull/3341)
 
 ## 0.29.1 (2026-06-18)