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>

internal/chapters/chapters.go

@@ -38,6 +38,7 @@ type Lap = models.Lap
 type Chapter struct {
 	Kind          string `json:"kind"`
 	Title         string `json:"title"`
+	Headline      string `json:"headline"`
 	StartLap      int    `json:"start_lap"`
 	EndLap        int    `json:"end_lap"`
 	StartTime     string `json:"start_time,omitempty"`