perf: cheaper ingest — no re-parse of live hooked transcripts, delta hook writes, lighter pollers

- Session sync leaves a transcript alone while hooks feed its session live
  (active, newest event a hook's, touched within DASHBOARD_SYNC_LIVE_HOOK_MS,
  default 5 min) and catches it up once quiet; a hook-created session (which
  importSession skips anyway) is only snapshotted, never parsed.
- Hook writer keeps a per-transcript mark (applied after commit): an
  unchanged TranscriptCache result skips token/compaction/API-error/turn
  writes, a grown one walks only the entries after the last one handled.
- Codex usage: one stamp query per pass instead of one row per journal;
  between full passes (first + every 5 min) only the newest day folders and
  journals written in the last hour are statted; fork parents resolved from
  the table or a full walk.
- Cursor/Antigravity polls back off to 60 s after idle passes
  (server/lib/backoff-poll.js); lane sync runs only while another dashboard
  shares the data dir (checked every 30 s).

server/lib/backoff-poll.js

@@ -0,0 +1,43 @@
+/**
+ * @file A background poll that slows down while it finds nothing to do: the
+ * next pass runs `baseMs` after a pass that did work (or failed), `idleMs`
+ * after one whose `tick` resolved `false` (nothing changed, nothing pending).
+ * One pass at a time — the next is scheduled only once the last settled — and
+ * every timer is `unref`'d. Used by the Cursor and Antigravity usage polls.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+/** Default slow interval: 60 s, never faster than the base one. */
+const idleDefault = (baseMs) => Math.max(baseMs, 60_000);
+
+/**
+ * Run `tick` now, then again and again. `tick` resolves `false` for an idle
+ * pass; anything else (or a throw) keeps the base pace.
+ * @returns {{stop: () => void}}
+ */
+function startBackoffPoll(tick, { baseMs, idleMs = idleDefault(baseMs) }) {
+  let timer = null;
+  let stopped = false;
+  const schedule = (ms) => {
+    if (stopped) return;
+    timer = setTimeout(run, ms);
+    if (timer.unref) timer.unref();
+  };
+  function run() {
+    Promise.resolve()
+      .then(tick)
+      .then(
+        (busy) => schedule(busy === false ? idleMs : baseMs),
+        () => schedule(baseMs)
+      );
+  }
+  run();
+  return {
+    stop() {
+      stopped = true;
+      clearTimeout(timer);
+    },
+  };
+}
+
+module.exports = { startBackoffPoll, idleDefault };