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).

client/src/i18n/locales/en/settings.json

@@ -1217,6 +1217,23 @@
     "ignoreHint": "One absolute path per line. Hooks from these folders, and anything under them, are dropped whole; history import and the Codex reader skip their sessions. AGENTHUB_IGNORE_CWD sets the same list.",
     "envOwned": "A value here is set by the environment the dashboard started in, and cannot be changed from this page."
   },
+  "retention": {
+    "title": "Automatic cleanup",
+    "description": "Off by default. When on, the hourly maintenance deletes finished sessions (completed, error, abandoned) that started more than this many days ago — with their events, agents and token usage — a little at a time.",
+    "days": "Delete sessions older than",
+    "off": "Automatic cleanup is off.",
+    "on": "Sessions older than {{days}} days are deleted every hour.",
+    "invalid": "A whole number from 0 (off) to {{max}}",
+    "saved": "Automatic cleanup saved",
+    "lastRun": "Last run {{time}}: {{n}} session(s) deleted",
+    "space": "Database {{size}} · {{free}} free inside it · auto-vacuum: {{mode}}",
+    "compact": "Compact now",
+    "compacting": "Compacting…",
+    "compactConfirmTitle": "Compact the database?",
+    "compactConfirmBody": "This rewrites the whole database file to give free space back to the disk and turns on incremental auto-vacuum. On a large database it takes a while, and new hook events wait until it finishes.",
+    "compactDone": "Database compacted: {{before}} → {{after}}",
+    "compactFailed": "Compact failed: {{error}}"
+  },
   "backup": {
     "title": "Daily backup",
     "description": "A full copy of the database once a day, into a folder you choose — best on another disk, so a failed drive does not take the backups with it.",