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).

.env.example

@@ -72,9 +72,21 @@
 #   DASHBOARD_SESSION_SYNC_MS=30000
 
 # Hourly database upkeep: a non-blocking WAL checkpoint (the WAL then shrinks
-# to 64 MB) and PRAGMA optimize. 0 turns it off. Never VACUUMs.
+# to 64 MB), PRAGMA optimize, the opt-in retention below and a bounded
+# incremental_vacuum. 0 turns it off. Never a full VACUUM (that is Settings →
+# Data → Compact, on a click).
 #   DASHBOARD_DB_MAINTAIN_MS=3600000
 
+# Automatic retention (OFF unless set): finished sessions that started more
+# than N days ago (1–3650) are purged by the hourly upkeep. Usually set from
+# Settings → Data → Session cleanup, which writes it to the settings file.
+#   AGENTHUB_RETENTION_DAYS=90
+
+# Update check interval (ms, default 3600000 = 1 h, minimum 60000). A tick
+# with no browser connected is skipped; the next browser to connect gets one.
+# DASHBOARD_UPDATE_CHECK=0 turns the scheduled check off.
+#   DASHBOARD_UPDATE_CHECK_INTERVAL_MS=3600000
+
 # Poll interval (ms) for Antigravity CLI sessions and tokens, read from agy's
 # own ~/.gemini/antigravity-cli/conversations/*.db (0 disables; default: 15000).
 #   DASHBOARD_ANTIGRAVITY_POLL_MS=15000