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).

docs/API.md

@@ -3057,6 +3057,16 @@ POST   /api/settings/backup/run    202 — started (or already running); poll GE
 Settings → Data → **Daily backup** (`server/lib/db-backup.js`). Once a day, at `hour` (0–23, the server's local time, default 2), the database is copied to `dir` as `agenthub-YYYYMMDD-HHMMSS.db`: `VACUUM INTO` on a read-only connection in a worker thread, so the copy is a consistent snapshot (writes from another dashboard on the same database never restart it), compacted, and the event loop keeps serving while it runs (≈30 s for a 1.25 GB database). It is written as `<name>.partial` and renamed when complete. Beyond `keep` (1–365, default 7) the oldest are deleted — only regular files matching that exact name; nothing else in the folder, and no symlink, is ever touched. A day missed while the server was down runs at the next start; a failed run is retried the next day or by **Back up now**, never every minute. The three values persist to the settings file as `AGENTHUB_BACKUP_DIR` / `AGENTHUB_BACKUP_HOUR` / `AGENTHUB_BACKUP_KEEP`; one the environment set is `env_owned` (`409 EENVOWNED`). Writes are same-origin, and admin-only in server mode. `400 INVALID_PATH` for a relative, missing, non-directory or unwritable folder, `400 INVALID_INPUT` for an hour or count out of range, `400 ENOBACKUPDIR` for a run with no folder. The scheduler beat is `DASHBOARD_BACKUP_TICK_MS` (default 60000, `0` = off).
 
 Restoring is manual, on purpose: `agenthub stop`, delete `dashboard.db-wal` and `dashboard.db-shm`, copy a backup over `dashboard.db` (its path is on Settings → System), `agenthub start`.
+
+### Retention and Compact
+
+```
+GET    /api/settings/retention     { days, max_days, env_owned: { days }, last_run, space: { auto_vacuum, page_size, page_count, freelist_pages, free_bytes }, db_size, compacting, last_compact }
+PUT    /api/settings/retention     { days } → the same state; 0 turns it off (the default)
+POST   /api/settings/compact       { confirm: true } → 202 — started (or already running); poll GET while `compacting`
+```
+
+Settings → Data → **Session cleanup** (`server/lib/db-retention.js`). Automatic retention is OFF unless `days` (1–3650, `AGENTHUB_RETENTION_DAYS` in the settings file) is set; nothing is deleted by default. When set, the hourly maintenance (`DASHBOARD_DB_MAINTAIN_MS`) purges finished (completed/error/abandoned) sessions that started more than `days` ago — the same rule and the same deletion (`purgeSessionIds`: events, agents, token usage, the session) as `POST /api/settings/cleanup` with `purge_days` — 25 sessions per transaction with a yield between, at most 5000 per run. A new database is created with `auto_vacuum = INCREMENTAL`, an existing one up to 32 MB is converted by a VACUUM at startup; the same hourly run then returns free pages to the OS (`incremental_vacuum`, in 1024-page slices, at most 65536 pages per run) once more than 1024 are free. A larger, older database is converted only by **Compact**: a full VACUUM in a worker thread followed by a TRUNCATE checkpoint — it holds the write lock while it runs (tens of seconds on a GB-size file), so hooks wait meanwhile; it runs only on a click. In server mode both writes are an admin's (`/settings` is in `ADMIN_WRITE_PREFIXES`).
 ---
 
 ## WebSocket API