fix: resolve CI lint errors and test compilation failures for rate limit feature

- Fix errcheck: properly handle rows.Close() error via named return + defer closure
- Fix gofmt: auto-format billing_cache.go, api_key_service.go, billing_cache_service.go
- Add missing rate limit interface methods to 4 test stubs (GetRateLimitData, IncrementRateLimitUsage, ResetRateLimitWindows)
- Fix NewBillingCacheService calls missing the new apiKeyRepo parameter

backend/internal/service/billing_cache_service.go

@@ -83,12 +83,12 @@ type apiKeyRateLimitLoader interface {
 // BillingCacheService 计费缓存服务
 // 负责余额和订阅数据的缓存管理，提供高性能的计费资格检查
 type BillingCacheService struct {
-	cache                  BillingCache
-	userRepo               UserRepository
-	subRepo                UserSubscriptionRepository
-	apiKeyRateLimitLoader  apiKeyRateLimitLoader
-	cfg                    *config.Config
-	circuitBreaker         *billingCircuitBreaker
+	cache                 BillingCache
+	userRepo              UserRepository
+	subRepo               UserSubscriptionRepository
+	apiKeyRateLimitLoader apiKeyRateLimitLoader
+	cfg                   *config.Config
+	circuitBreaker        *billingCircuitBreaker
 
 	cacheWriteChan     chan cacheWriteTask
 	cacheWriteWg       sync.WaitGroup
@@ -106,11 +106,11 @@ type BillingCacheService struct {
 // NewBillingCacheService 创建计费缓存服务
 func NewBillingCacheService(cache BillingCache, userRepo UserRepository, subRepo UserSubscriptionRepository, apiKeyRepo APIKeyRepository, cfg *config.Config) *BillingCacheService {
 	svc := &BillingCacheService{
-		cache:                  cache,
-		userRepo:               userRepo,
-		subRepo:                subRepo,
-		apiKeyRateLimitLoader:  apiKeyRepo,
-		cfg:                    cfg,
+		cache:                 cache,
+		userRepo:              userRepo,
+		subRepo:               subRepo,
+		apiKeyRateLimitLoader: apiKeyRepo,
+		cfg:                   cfg,
 	}
 	svc.circuitBreaker = newBillingCircuitBreaker(cfg.Billing.CircuitBreaker)
 	svc.startCacheWriteWorkers()