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/index.js

@@ -825,18 +825,35 @@ function startBackgroundServices() {
   }
   // Another dashboard on the same database (hooks reach only one of them)
   // moving a lane: re-broadcast it here. One pragma per beat when nothing
-  // changed. Off with DASHBOARD_LANE_SYNC_MS=0.
+  // changed — and only while such a dashboard runs at all: alone on its data
+  // dir, this server only looks for one every LANE_PEER_CHECK_MS (the first
+  // call primes what it has seen, so a peer that starts later is compared
+  // against it). Off with DASHBOARD_LANE_SYNC_MS=0.
   const LANE_SYNC_MS = Number(process.env.DASHBOARD_LANE_SYNC_MS ?? 2_000);
   if (Number.isFinite(LANE_SYNC_MS) && LANE_SYNC_MS > 0) {
     const { broadcastForeignLaneChanges } = require("./routes/lanes");
-    const laneSyncTimer = setInterval(() => {
+    const LANE_PEER_CHECK_MS = Math.max(LANE_SYNC_MS, 30_000);
+    let peersSeenAt = 0;
+    let hasPeers = false;
+    const syncLanes = () => {
       try {
         broadcastForeignLaneChanges();
       } catch (err) {
         console.warn("lane sync failed:", err.message);
       }
-    }, LANE_SYNC_MS);
-    if (laneSyncTimer.unref) laneSyncTimer.unref();
+    };
+    syncLanes();
+    const laneSyncTick = () => {
+      if (Date.now() - peersSeenAt >= LANE_PEER_CHECK_MS) {
+        hasPeers = peersSharingDataDir().length > 0;
+        peersSeenAt = Date.now();
+      }
+      if (hasPeers) syncLanes();
+      const t = setTimeout(laneSyncTick, hasPeers ? LANE_SYNC_MS : LANE_PEER_CHECK_MS);
+      if (t.unref) t.unref();
+    };
+    const first = setTimeout(laneSyncTick, LANE_SYNC_MS);
+    if (first.unref) first.unref();
   }
   // Continuous discovery of sessions under ~/.claude/projects. The one-time
   // legacy backfill above runs only once (marker-gated), so a project added