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/__tests__/Settings.test.tsx

@@ -77,6 +77,18 @@ vi.mock("../../lib/api", async (importOriginal) => {
             env_owned: { dir: false, hour: false, keep: false },
           })),
         },
+        retention: {
+          get: vi.fn(async () => ({
+            days: 0,
+            max_days: 3650,
+            env_owned: { days: false },
+            last_run: null,
+            space: null,
+            db_size: 0,
+            compacting: false,
+            last_compact: null,
+          })),
+        },
       },
     },
   };