feat: implement Yandex /search method for on-demand edge expansion (Task 2)

- Add SearchRoutes method to yandex client for on-demand station pair searches
- Implement hub expansion via on-demand /search calls in lazy graph expansion
- Integrate cache key generation for search results (search:{from}:{to}:{date})
- Update expandFromStation and expandFromCityHub to use Yandex API
- Add NewGraphWithoutYandex constructor for testability
- Add tests for on-demand search integration and cache TTL policies

cmd/api/main.go

@@ -14,8 +14,8 @@ import (
 
 func main() {
 	redisClient := initRedis()
-	router := routing.NewGraph()
 	yandexClient := yandex.NewClient("default-key")
+	router := routing.NewGraph(yandexClient)
 
 	handlerCtx := NewHandlerContext(redisClient, router, yandexClient)