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/pages/Settings.tsx

@@ -1,6 +1,6 @@
 /**
  * @file Settings.tsx
- * @description The install-wide settings, as three pages of the Settings nav (`SettingsHub.tsx`): `section="data"` — total cost, database overview, workspace housekeeping (dry run first, confirm before a real run), import, remote sources, push ingest (`components/settings/PushIngestSettings.tsx`), privacy (`components/settings/PrivacySettings.tsx`), the daily database backup (`components/settings/BackupSettings.tsx`), export, this browser's preferences backup, session cleanup (whole days ≥ 1, a dry-run count in its confirm) and the danger zone (typed confirmation) —, `section="about"` (the System page) — the start-at-login switch and Quit (`AutostartSettings`), then the server process (release, uptime, platform, CPU, memory, live connections), how AgentHub is installed (the update check's cached status) and the database file — and `section="notifications"` — browser notifications (per-browser, with a reset) and alerts (server-wide). Server info (`/api/settings/info`) is read only by Data and About, refreshed on activity at most every 10 s. Language, theme and the pet live on the Appearance page (`components/settings/AppearanceSettings.tsx`); per-tool settings live in that tool's page under `/settings/agents/<id>`.
+ * @description The install-wide settings, as three pages of the Settings nav (`SettingsHub.tsx`): `section="data"` — total cost, database overview, workspace housekeeping (dry run first, confirm before a real run), import, remote sources, push ingest (`components/settings/PushIngestSettings.tsx`), privacy (`components/settings/PrivacySettings.tsx`), the daily database backup (`components/settings/BackupSettings.tsx`), export, this browser's preferences backup, session cleanup (whole days ≥ 1, a dry-run count in its confirm; under it the opt-in automatic cleanup and Compact, `components/settings/RetentionSettings.tsx`) and the danger zone (typed confirmation) —, `section="about"` (the System page) — the start-at-login switch and Quit (`AutostartSettings`), then the server process (release, uptime, platform, CPU, memory, live connections), how AgentHub is installed (the update check's cached status) and the database file — and `section="notifications"` — browser notifications (per-browser, with a reset) and alerts (server-wide). Server info (`/api/settings/info`) is read only by Data and About, refreshed on activity at most every 10 s. Language, theme and the pet live on the Appearance page (`components/settings/AppearanceSettings.tsx`); per-tool settings live in that tool's page under `/settings/agents/<id>`.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 /* =============================================================================
@@ -103,6 +103,7 @@ import { RemoteSources } from "../components/RemoteSources";
 import { PushIngestSettings } from "../components/settings/PushIngestSettings";
 import { PrivacySettings } from "../components/settings/PrivacySettings";
 import { BackupSettings } from "../components/settings/BackupSettings";
+import { RetentionSettings } from "../components/settings/RetentionSettings";
 import { AutostartSettings } from "../components/settings/AutostartSettings";
 import { usePermissions } from "../components/AuthGate";
 import { Skeleton } from "../components/Skeleton";
@@ -1054,6 +1055,8 @@ export function Settings({
             </button>
 
             {actionBanner(["cleanup"])}
+
+            <RetentionSettings />
           </section>
 
           {/* ─── THIS BROWSER'S PREFERENCES ─── */}