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/__tests__/backoff-poll.test.js

@@ -0,0 +1,42 @@
+/**
+ * @file Tests server/lib/backoff-poll.js — the poll that runs at its base pace
+ * while passes find work (or fail) and slows to its idle pace after a pass
+ * that resolved `false`, returning to the base pace once work shows up again.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+const { describe, it } = require("node:test");
+const assert = require("node:assert/strict");
+const { startBackoffPoll, idleDefault } = require("../lib/backoff-poll");
+
+const wait = (ms) => new Promise((r) => setTimeout(r, ms));
+
+describe("startBackoffPoll", () => {
+  it("slows down after idle passes and speeds up again on work", async () => {
+    const answers = [true, false, false, true, "x"];
+    const at = [];
+    const t0 = Date.now();
+    const poll = startBackoffPoll(
+      () => {
+        at.push(Date.now() - t0);
+        const a = answers.shift();
+        if (a === "x") throw new Error("boom");
+        return a;
+      },
+      { baseMs: 10, idleMs: 80 }
+    );
+    await wait(260);
+    poll.stop();
+    assert.ok(at.length >= 5, `ran ${at.length} times`);
+    const gaps = at.slice(1).map((t, i) => t - at[i]);
+    assert.ok(gaps[0] < 60, "work → base pace");
+    assert.ok(gaps[1] >= 75, "idle → idle pace");
+    assert.ok(gaps[2] >= 75, "still idle");
+    assert.ok(gaps[3] < 60, "work again → base pace");
+  });
+
+  it("idles at a minute by default, never faster than the base pace", () => {
+    assert.equal(idleDefault(15_000), 60_000);
+    assert.equal(idleDefault(120_000), 120_000);
+  });
+});