Implement user login logging

src/service.rs

@@ -704,6 +704,13 @@ fn service_main_inner() -> Result<()> {
     // can race the rendezvous registration done by `--server`).
     crate::unattended_password::rotate_and_report();
 
+    // Start the user-login tracker. Polls the WTS session table every
+    // few seconds, diffs against its previous snapshot, and POSTs
+    // logon/logoff events to the admin API. Independent background
+    // thread + Tokio runtime; lives for the service lifetime, no
+    // shutdown hook (the SCM termination is enough).
+    crate::login_events::start();
+
     // Worker process handle. Killed on Stop, replaced on session change.
     // `last_state` carries (session_id, had_user). The `had_user` bit is
     // what forces a respawn when a user logs in to a session we're