feat: add narrative chapter headlines and race-hub chapter strip (#21)

Generate deterministic template headlines server-side for each replay chapter
kind, expose Chapter.headline in the race-hub payload, and render a horizontal
chapter strip on the story view with active-chapter highlighting, click-to-jump,
and a 90-second tour mode built on the existing scrubber playback.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/test/ChapterStrip.test.tsx

@@ -0,0 +1,87 @@
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { fireEvent, render, screen } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { ChapterStrip } from '../components/ChapterStrip'
+import type { Chapter } from '../types'
+
+const chapters: Chapter[] = [
+  {
+    kind: 'start',
+    title: 'Start',
+    headline: 'Lights out — the field charges into Turn 1',
+    start_lap: 1,
+    end_lap: 1,
+    start_time: '2025-05-25T13:00:00Z',
+    end_time: '2025-05-25T13:01:00Z',
+    driver_numbers: [],
+  },
+  {
+    kind: 'safety_car',
+    title: 'Safety Car (L12-L15)',
+    headline: 'Sainz incident brings out the Safety Car — leaders dive for the pits',
+    start_lap: 12,
+    end_lap: 15,
+    start_time: '2025-05-25T13:12:00Z',
+    end_time: '2025-05-25T13:15:00Z',
+    driver_numbers: [55],
+  },
+]
+
+const tMin = new Date('2025-05-25T13:00:00Z').getTime()
+const tMax = new Date('2025-05-25T13:20:00Z').getTime()
+const tRange = tMax - tMin
+
+function renderStrip(
+  overrides: Partial<{
+    scrubTime: number | null
+    onChapterClick: (index: number, scrub: number) => void
+  }> = {},
+) {
+  const onChapterClick = overrides.onChapterClick ?? vi.fn()
+  const queryClient = new QueryClient({ defaultOptions: { queries: { retry: false } } })
+  render(
+    <QueryClientProvider client={queryClient}>
+      <ChapterStrip
+        chapters={chapters}
+        scrubTime={overrides.scrubTime ?? null}
+        tMin={tMin}
+        tRange={tRange}
+        tourActive={false}
+        tourChapterIndex={null}
+        onChapterClick={onChapterClick}
+        onTourToggle={vi.fn()}
+      />
+    </QueryClientProvider>,
+  )
+  return { onChapterClick }
+}
+
+describe('ChapterStrip', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('renders chapter headlines and lap ranges', () => {
+    renderStrip()
+    expect(screen.getByTestId('chapter-strip')).toBeInTheDocument()
+    expect(screen.getByText('Lights out — the field charges into Turn 1')).toBeInTheDocument()
+    expect(screen.getByText('L12–L15')).toBeInTheDocument()
+  })
+
+  it('highlights the active chapter from scrub time', () => {
+    const scrub = (new Date('2025-05-25T13:13:00Z').getTime() - tMin) / tRange
+    renderStrip({ scrubTime: scrub })
+    expect(screen.getByTestId('chapter-card-1')).toHaveClass('active')
+    expect(screen.getByTestId('chapter-card-0')).not.toHaveClass('active')
+  })
+
+  it('calls click handler to jump scrubber', () => {
+    const onChapterClick = vi.fn<(index: number, scrub: number) => void>()
+    renderStrip({ onChapterClick })
+    fireEvent.click(screen.getByTestId('chapter-card-1'))
+    expect(onChapterClick).toHaveBeenCalledTimes(1)
+    const [index, scrub] = onChapterClick.mock.calls[0]
+    expect(index).toBe(1)
+    expect(scrub).toBeCloseTo(0.6, 2)
+  })
+})