feat: add replay track map scrubber

Spike: replay location fan-out is bounded with a semaphore at 4 concurrent GetLocation calls, covered by TestAssembleReplayFramesBoundsLocationFanOut.

Adds /api/v1/replay/frames with 5s interval floor, 3000-frame cap, nearest-sample snapping, and empty-driver omission. Wires RaceStoryCanvas to lazy-load replay frames plus cached track outlines when the map panel opens, sharing the existing scrubber/playback state across chart and map.

Tests: HOME=/private/tmp/box-box-home GOCACHE=/private/tmp/box-box-go-cache GOMODCACHE=/Users/aman/go/pkg/mod go test ./internal/web; go build ./... (same env, passed with read-only module stat-cache warning); npm run test; npx tsc --noEmit. Full go test ./... is blocked in this sandbox by existing network/listener-dependent internal/api and internal/news tests.

frontend/src/types.ts

@@ -360,6 +360,23 @@ export interface TrackOutline {
   bounds: TrackBounds
 }
 
+export interface ReplayCarPosition {
+  x: number
+  y: number
+}
+
+export interface ReplayFrame {
+  t: number
+  cars: Record<string, ReplayCarPosition>
+}
+
+export interface ReplayFramesResponse {
+  session_key: number
+  interval_ms: number
+  start_time: string
+  frames: ReplayFrame[]
+}
+
 export interface ChampHubDriver {
   driver_number: number
   name_acronym: string