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/server/middleware/api_key_auth_test.go

@@ -588,6 +588,16 @@ func (r *stubApiKeyRepo) UpdateLastUsed(ctx context.Context, id int64, usedAt ti
 	return nil
 }
 
+func (r *stubApiKeyRepo) IncrementRateLimitUsage(ctx context.Context, id int64, cost float64) error {
+	return nil
+}
+func (r *stubApiKeyRepo) ResetRateLimitWindows(ctx context.Context, id int64) error {
+	return nil
+}
+func (r *stubApiKeyRepo) GetRateLimitData(ctx context.Context, id int64) (*service.APIKeyRateLimitData, error) {
+	return nil, nil
+}
+
 type stubUserSubscriptionRepo struct {
 	getActive      func(ctx context.Context, userID, groupID int64) (*service.UserSubscription, error)
 	updateStatus   func(ctx context.Context, subscriptionID int64, status string) error