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/antigravity-usage.js

@@ -425,6 +425,9 @@ function writeSession(f, conv, now) {
 }
 
 let running = null;
+// Fresh sessions the last pass left for a later one to settle (the poll keeps
+// its base pace while there are any).
+let lastPending = 0;
 
 /**
  * One pass over every conversation file: a changed one is re-read, an
@@ -440,12 +443,16 @@ function ingestAntigravityUsage({
   running = (async () => {
     let read = 0;
     let written = 0;
+    let pending = 0; // still-fresh sessions a later pass may have to settle
     for (const f of conversationFiles()) {
       try {
         const seen = getSeen.get(f.path);
         if (seen && seen.mtime_ms === f.mtime) {
           const a = activity(f.id, f.mtime);
-          if (fresh(a, now)) continue;
+          if (fresh(a, now)) {
+            pending += 1;
+            continue;
+          }
           const s = stmts.getSession.get(f.id);
           if (s && s.tool === "antigravity" && s.status === "active") {
             const at = new Date(a.last).toISOString();
@@ -486,6 +493,7 @@ function ingestAntigravityUsage({
       }
       await new Promise((r) => setImmediate(r));
     }
+    lastPending = pending;
     return { files: read, sessions: written };
   })().finally(() => {
     running = null;
@@ -504,16 +512,16 @@ function startAntigravityUsagePoll(broadcast, broadcastLane) {
   if (!Number.isFinite(POLL_MS) || POLL_MS <= 0) return;
   // Files an older build could not read get another try.
   db.prepare("DELETE FROM antigravity_conversations WHERE session_id = ''").run();
-  const tick = () => {
-    ingestAntigravityUsage({ broadcast, broadcastLane })
-      .then((r) => {
+  // Slowed to once a minute after a pass that read nothing, wrote nothing and
+  // left no fresh session to settle; back to POLL_MS once one finds work.
+  require("./backoff-poll").startBackoffPoll(
+    () =>
+      ingestAntigravityUsage({ broadcast, broadcastLane }).then((r) => {
         if (r.sessions > 0 && pricingSync.syncEnabled()) void pricingSync.syncPricing(db);
-      })
-      .catch(() => {});
-  };
-  tick();
-  const timer = setInterval(tick, POLL_MS);
-  if (timer.unref) timer.unref();
+        return r.files + r.sessions + lastPending > 0;
+      }),
+    { baseMs: POLL_MS }
+  );
 }
 
 module.exports = {