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/airports/airports.go

@@ -35,9 +35,9 @@ type StationNeighbors struct {
 // NewStationNeighbors creates a new StationNeighbors instance for the given city code.
 func NewStationNeighbors(cityCode string) *StationNeighbors {
 	return &StationNeighbors{
-		CityCode: cityCode,
+		CityCode:  cityCode,
 		Neighbors: []StationNeighbor{},
-		byID:     make(map[string]int),
+		byID:      make(map[string]int),
 	}
 }
 
@@ -97,4 +97,4 @@ func (sn *StationNeighbors) GetNonExcluded() []StationNeighbor {
 		}
 	}
 	return result
-}
+}