fix(story-mode): live helper steps behind an unchanged page, a clean reopen, no wedged view

From review round 7 (PR review):

- A helper's drawer froze while the helper worked: a background helper
  writes only to its own transcript, so the parent page answered 304 and
  the refresh returned before anything refreshed the drawer. An unchanged
  page now still refreshes an open drawer's working helpers (ETag'd).
- A helper messaged twice read as superseded ("The trail stops here…"):
  its one-page run took its ending from the first turn. It takes the last.
- More than 60 turns in a small file listed the newest 60 with no hint;
  `older` is now set whenever the list leaves turns out.
- One /story request that never settled wedged the view, even across a
  reopen. Requests carry a token and a 30 s timeout, and opening resets
  the request state, with what was announced and shown.
- The background canvases (megabytes each at 2x) are released when Story
  Mode closes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/routes/story-routes.test.ts

@@ -202,6 +202,16 @@ describe('GET /api/sessions/:id/story', () => {
     expect(res.json().data).toMatchObject({ supported: true, turns: [], page: null });
   });
 
+  it('says when the list leaves turns out past the 60 it shows, even in a small file', async () => {
+    const lines: unknown[] = [];
+    for (let i = 0; i < 80; i++) lines.push(prompt(`job ${i}`), said('ok'), end(1000));
+    writeFileSync(transcript, jsonl(lines));
+    const { app } = await setup();
+    const body = (await app.inject({ method: 'GET', url: URL })).json();
+    expect(body.data.turns).toHaveLength(60);
+    expect(body.data.older).toBe(true);
+  });
+
   it('404s an unknown session', async () => {
     const { app } = await setup();
     const res = await app.inject({ method: 'GET', url: '/api/sessions/nope/story' });