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/admin_service_delete_test.go

@@ -348,6 +348,19 @@ func (s *billingCacheStub) InvalidateSubscriptionCache(ctx context.Context, user
 	return nil
 }
 
+func (s *billingCacheStub) GetAPIKeyRateLimit(ctx context.Context, keyID int64) (*APIKeyRateLimitCacheData, error) {
+	panic("unexpected GetAPIKeyRateLimit call")
+}
+func (s *billingCacheStub) SetAPIKeyRateLimit(ctx context.Context, keyID int64, data *APIKeyRateLimitCacheData) error {
+	panic("unexpected SetAPIKeyRateLimit call")
+}
+func (s *billingCacheStub) UpdateAPIKeyRateLimitUsage(ctx context.Context, keyID int64, cost float64) error {
+	panic("unexpected UpdateAPIKeyRateLimitUsage call")
+}
+func (s *billingCacheStub) InvalidateAPIKeyRateLimit(ctx context.Context, keyID int64) error {
+	panic("unexpected InvalidateAPIKeyRateLimit call")
+}
+
 func waitForInvalidations(t *testing.T, ch <-chan subscriptionInvalidateCall, expected int) []subscriptionInvalidateCall {
 	t.Helper()
 	calls := make([]subscriptionInvalidateCall, 0, expected)