feat: image output token billing, channel-mapped billing source, credits balance precheck

- Parse candidatesTokensDetails from Gemini API to separate image/text output tokens
- Add image_output_tokens and image_output_cost to usage_log (migration 089)
- Support per-image-token pricing via output_cost_per_image_token from model pricing data
- Channel pricing ImageOutputPrice override works in token billing mode
- Auto-fill image_output_price in channel pricing form from model defaults
- Add "channel_mapped" billing model source as new default (migration 088)
- Bills by model name after channel mapping, before account mapping
- Fix channel cache error TTL sign error (115s → 5s)
- Fix Update channel only invalidating new groups, not removed groups
- Fix frontend model_mapping clearing sending undefined instead of {}
- Credits balance precheck via shared AccountUsageService cache before injection
- Skip credits injection for accounts with insufficient balance
- Don't mark credits exhausted for "exhausted your capacity on this model" 429s

backend/internal/service/gateway_service.go

@@ -483,6 +483,7 @@ type ClaudeUsage struct {
 	CacheReadInputTokens     int `json:"cache_read_input_tokens"`
 	CacheCreation5mTokens    int // 5分钟缓存创建token（来自嵌套 cache_creation 对象）
 	CacheCreation1hTokens    int // 1小时缓存创建token（来自嵌套 cache_creation 对象）
+	ImageOutputTokens        int `json:"image_output_tokens,omitempty"`
 }
 
 // ForwardResult 转发结果
@@ -7729,6 +7730,9 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 	var cost *CostBreakdown
 	// 确定计费模型
 	billingModel := forwardResultBillingModel(result.Model, result.UpstreamModel)
+	if input.BillingModelSource == BillingModelSourceChannelMapped && input.ChannelMappedModel != "" {
+		billingModel = input.ChannelMappedModel
+	}
 	if input.BillingModelSource == BillingModelSourceRequested && input.OriginalModel != "" {
 		billingModel = input.OriginalModel
 	}
@@ -7777,6 +7781,7 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 			CacheReadTokens:       result.Usage.CacheReadInputTokens,
 			CacheCreation5mTokens: result.Usage.CacheCreation5mTokens,
 			CacheCreation1hTokens: result.Usage.CacheCreation1hTokens,
+			ImageOutputTokens:     result.Usage.ImageOutputTokens,
 		}
 		var err error
 		if s.resolver != nil && apiKey.Group != nil {
@@ -7836,8 +7841,10 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 		CacheReadTokens:       result.Usage.CacheReadInputTokens,
 		CacheCreation5mTokens: result.Usage.CacheCreation5mTokens,
 		CacheCreation1hTokens: result.Usage.CacheCreation1hTokens,
+		ImageOutputTokens:     result.Usage.ImageOutputTokens,
 		InputCost:             cost.InputCost,
 		OutputCost:            cost.OutputCost,
+		ImageOutputCost:       cost.ImageOutputCost,
 		CacheCreationCost:     cost.CacheCreationCost,
 		CacheReadCost:         cost.CacheReadCost,
 		TotalCost:             cost.TotalCost,
@@ -7976,6 +7983,9 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 	var cost *CostBreakdown
 	// 确定计费模型
 	billingModel := forwardResultBillingModel(result.Model, result.UpstreamModel)
+	if input.BillingModelSource == BillingModelSourceChannelMapped && input.ChannelMappedModel != "" {
+		billingModel = input.ChannelMappedModel
+	}
 	if input.BillingModelSource == BillingModelSourceRequested && input.OriginalModel != "" {
 		billingModel = input.OriginalModel
 	}
@@ -8007,6 +8017,7 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 			CacheReadTokens:       result.Usage.CacheReadInputTokens,
 			CacheCreation5mTokens: result.Usage.CacheCreation5mTokens,
 			CacheCreation1hTokens: result.Usage.CacheCreation1hTokens,
+			ImageOutputTokens:     result.Usage.ImageOutputTokens,
 		}
 		var err error
 		// 优先尝试 Resolver + CalculateCostUnified（仅在有渠道定价时使用）
@@ -8073,8 +8084,10 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 		CacheReadTokens:       result.Usage.CacheReadInputTokens,
 		CacheCreation5mTokens: result.Usage.CacheCreation5mTokens,
 		CacheCreation1hTokens: result.Usage.CacheCreation1hTokens,
+		ImageOutputTokens:     result.Usage.ImageOutputTokens,
 		InputCost:             cost.InputCost,
 		OutputCost:            cost.OutputCost,
+		ImageOutputCost:       cost.ImageOutputCost,
 		CacheCreationCost:     cost.CacheCreationCost,
 		CacheReadCost:         cost.CacheReadCost,
 		TotalCost:             cost.TotalCost,