perf: hourly update check only with a browser open; opt-in retention, incremental auto-vacuum and Compact

- update-scheduler: default interval 1 h (DASHBOARD_UPDATE_CHECK_INTERVAL_MS and its 60 s floor kept);
  a scheduled tick with no /ws client is skipped, and the next browser to connect gets one catch-up
  check when the last one is stale (websocket.js onConnection). Manual check unchanged.
- db-retention.js: shared session purge (used by POST /api/settings/cleanup too), opt-in
  AGENTHUB_RETENTION_DAYS applied by the hourly maintenance in 25-session transactions with yields
  (max 5000/run), bounded incremental_vacuum when the freelist is large, and a manual Compact
  (VACUUM in a worker, converting to auto_vacuum=INCREMENTAL).
- db.js: new databases are INCREMENTAL; an existing one up to 32 MB is converted at startup.
- GET/PUT /api/settings/retention, POST /api/settings/compact (confirm: true); Settings → Data →
  Session cleanup gets the Automatic cleanup control and Compact now (en + vi).

server/websocket.js

@@ -48,6 +48,13 @@ function initWebSocket(server) {
   });
 
   wss.on("connection", (ws, req) => {
+    for (const fn of connectionListeners) {
+      try {
+        fn(ws);
+      } catch {
+        /* a listener never breaks a connection */
+      }
+    }
     // Server mode: who this socket belongs to, for broadcast filtering, and
     // the cookie to re-check it by (a logout or a disabled user ends it).
     if (isServerMode()) {
@@ -252,6 +259,16 @@ function broadcast(type, data) {
   });
 }
 
+/**
+ * Subscribe to new `/ws` browser connections (the update scheduler checks once
+ * when a browser returns after its ticks were skipped). Returns an unsubscribe.
+ */
+const connectionListeners = new Set();
+function onConnection(fn) {
+  connectionListeners.add(fn);
+  return () => connectionListeners.delete(fn);
+}
+
 function getConnectionCount() {
   if (!wss) return 0;
   let count = 0;
@@ -306,6 +323,7 @@ module.exports = {
   initPtyWebSocket,
   broadcast,
   getConnectionCount,
+  onConnection,
   closeWebSocket,
   sizeFromQuery,
 };