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/index.js

@@ -500,11 +500,17 @@ function startBackgroundServices() {
     .catch(() => {});
 
   // Hourly DB upkeep: a non-blocking checkpoint (the WAL then shrinks to
-  // journal_size_limit) and PRAGMA optimize. Off with DASHBOARD_DB_MAINTAIN_MS=0.
+  // journal_size_limit), PRAGMA optimize, the opt-in retention
+  // (AGENTHUB_RETENTION_DAYS, off by default) and a bounded incremental_vacuum
+  // (server/lib/db-retention.js). Off with DASHBOARD_DB_MAINTAIN_MS=0.
   {
     const every = Number(process.env.DASHBOARD_DB_MAINTAIN_MS ?? 60 * 60 * 1000);
     if (every > 0) {
-      const t = setInterval(() => require("./db").maintainDatabase(), every);
+      const t = setInterval(() => {
+        require("./lib/db-retention")
+          .runScheduledMaintenance()
+          .catch(() => {});
+      }, every);
       if (t.unref) t.unref();
     }
   }