fix: address audit findings - cache sync, validation, consistency

- clearCreditsExhausted: sync Redis scheduler cache after DB update
- Image billing mode UI: write to per_request_price instead of image_output_price
- OpenAI RecordUsage: use BillingModelSourceRequested constant, add s.cfg nil guard
- Fix i18n key path: admin.channels.perRequestPriceRequired → admin.channels.form.perRequestPriceRequired

backend/internal/service/openai_gateway_service.go

@@ -4182,7 +4182,10 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 	}
 
 	// Get rate multiplier
-	multiplier := s.cfg.Default.RateMultiplier
+	multiplier := 1.0
+	if s.cfg != nil {
+		multiplier = s.cfg.Default.RateMultiplier
+	}
 	if apiKey.GroupID != nil && apiKey.Group != nil {
 		resolver := s.userGroupRateResolver
 		if resolver == nil {
@@ -4200,7 +4203,7 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 	if input.BillingModelSource == BillingModelSourceChannelMapped && input.ChannelMappedModel != "" {
 		billingModel = input.ChannelMappedModel
 	}
-	if input.BillingModelSource == "requested" && input.OriginalModel != "" {
+	if input.BillingModelSource == BillingModelSourceRequested && input.OriginalModel != "" {
 		billingModel = input.OriginalModel
 	}
 	serviceTier := ""