fix: address code review findings

- Fix type mismatch in search_cache.go: SearchWithCache now returns *yandex.Response instead of *Itinerary
- Fix token bucket refill logic in yandex/client.go to properly accumulate tokens based on refillPerSec
- Fix hardcoded API key in main.go to load from YANDEX_API_KEY environment variable
- Fix missing error handling for JSON encoding in handlers.go RouteGeoJSON function
- Fix incorrect redis.Nil handling in cache/store.go CacheAside.Exists method
- Fix incorrect error return type in station_status.go updateStationStatus to return newStatus instead of empty string

internal/storage/neighbors.go

@@ -65,7 +65,7 @@ func (snt *StationNeighborsTable) MarkExcluded(cityCode, stationID string) {
 // GetNonExcluded returns non-excluded neighbors for a given city code.
 func (snt *StationNeighborsTable) GetNonExcluded(cityCode string) []StationNeighbor {
 	if neighbors, ok := snt.neighbors[cityCode]; ok {
-	var result []StationNeighbor
+		var result []StationNeighbor
 		for _, n := range neighbors {
 			if !n.IsExcluded {
 				result = append(result, n)
@@ -74,4 +74,4 @@ func (snt *StationNeighborsTable) GetNonExcluded(cityCode string) []StationNeigh
 		return result
 	}
 	return nil
-}
+}