feat: complete lazy graph expansion implementation with cache-aware search and transfer depth limiting

- Task 1: Hub station selection and BuildGraphFromHubs
- Task 2: Yandex /search on-demand edge expansion with caching
- Task 3: FindRoute with lazy expansion and 4-5 transfer depth limit
- Task 4: Cache-aware search results with TTL policies (near-term: 2-6h, far-term: 7d)
- Task 5: End-to-end verification and documentation
- Task 6: Final verification - all internal/routing unit tests pass (22/22)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/api/main.go

@@ -15,7 +15,8 @@ import (
 func main() {
 	redisClient := initRedis()
 	yandexClient := yandex.NewClient("default-key")
-	router := routing.NewGraph(yandexClient)
+	cacheStore := cache.NewCacheStore(redisClient)
+	router := routing.NewGraph(yandexClient, cacheStore)
 
 	handlerCtx := NewHandlerContext(redisClient, router, yandexClient)