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

cmd/cron/station_status_test.go

@@ -235,7 +235,6 @@ func TestProcessStation_Reactivation_AfterClosure(t *testing.T) {
 	}
 }
 
-
 // TestAutoClosureChronology verifies the chronology of auto-closure detection.
 // It tests that a station closes after exactly N=3 consecutive zero-trip days,
 // and that it reactivates when trips resume.
@@ -337,4 +336,3 @@ func TestAutoClosureChronology(t *testing.T) {
 		t.Errorf("expected zero days 0 after reactivation, got %d", zeroDays4)
 	}
 }
-