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/yandex/client.go

@@ -325,11 +325,15 @@ func (tb *tokenBucket) acquire() error {
 func (tb *tokenBucket) refill(now time.Time) {
 	elapsed := now.Sub(tb.lastRefill)
 	if elapsed >= time.Second {
-		// Refill tokens based on elapsed time and rate
-		tb.tokens = tb.capacity
+		tokensToAdd := int(elapsed.Seconds()) * tb.refillPerSec
+		if tb.tokens+tokensToAdd > tb.capacity {
+			tb.tokens = tb.capacity
+		} else {
+			tb.tokens += tokensToAdd
+		}
 		tb.lastRefill = now
 	}
-	// else: keep current tokens, will fully refill on next second boundary
+	// else: keep current tokens, will add on next refill
 }
 
 // --- Circuit Breaker ---
@@ -342,7 +346,6 @@ func newCircuitBreaker() *circuitBreaker {
 	}
 }
 
-
 func (cb *circuitBreaker) ResetCircuitBreaker() {
 	cb.mu.Lock()
 	defer cb.mu.Unlock()
@@ -420,4 +423,4 @@ func applyJitter(backoff time.Duration) time.Duration {
 func randFloat64() float64 {
 	// Use math/rand with a seed based on function call index for variability
 	return rand.Float64()
-}
+}