Bypass COOKIE file requirement in systemd service

Set RELEASE_COOKIE environment variable directly in systemd service
to prevent startup script from trying to read non-existent COOKIE file.

The environment variable takes precedence over file reading.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

nixos-module.nix

@@ -71,6 +71,8 @@ in
         SYSTEM_STATS_STATS_TOPIC = cfg.statsTopic;
         SYSTEM_STATS_COMMAND_TOPIC = cfg.commandTopic;
         SYSTEM_STATS_PUBLISH_INTERVAL = toString cfg.publishInterval;
+        # Override RELEASE_COOKIE to bypass file reading
+        RELEASE_COOKIE = "systant-bypass-cookie";
       };
 
       serviceConfig = {