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/openai_gateway_service.go

@@ -204,6 +204,7 @@ type OpenAIUsage struct {
 	OutputTokens             int `json:"output_tokens"`
 	CacheCreationInputTokens int `json:"cache_creation_input_tokens,omitempty"`
 	CacheReadInputTokens     int `json:"cache_read_input_tokens,omitempty"`
+	ImageOutputTokens        int `json:"image_output_tokens,omitempty"`
 }
 
 // OpenAIForwardResult represents the result of forwarding
@@ -4177,6 +4178,7 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		OutputTokens:        result.Usage.OutputTokens,
 		CacheCreationTokens: result.Usage.CacheCreationInputTokens,
 		CacheReadTokens:     result.Usage.CacheReadInputTokens,
+		ImageOutputTokens:   result.Usage.ImageOutputTokens,
 	}
 
 	// Get rate multiplier
@@ -4195,6 +4197,9 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 	if result.BillingModel != "" {
 		billingModel = strings.TrimSpace(result.BillingModel)
 	}
+	if input.BillingModelSource == BillingModelSourceChannelMapped && input.ChannelMappedModel != "" {
+		billingModel = input.ChannelMappedModel
+	}
 	if input.BillingModelSource == "requested" && input.OriginalModel != "" {
 		billingModel = input.OriginalModel
 	}
@@ -4255,8 +4260,10 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		OutputTokens:          result.Usage.OutputTokens,
 		CacheCreationTokens:   result.Usage.CacheCreationInputTokens,
 		CacheReadTokens:       result.Usage.CacheReadInputTokens,
+		ImageOutputTokens:     result.Usage.ImageOutputTokens,
 		InputCost:             cost.InputCost,
 		OutputCost:            cost.OutputCost,
+		ImageOutputCost:       cost.ImageOutputCost,
 		CacheCreationCost:     cost.CacheCreationCost,
 		CacheReadCost:         cost.CacheReadCost,
 		TotalCost:             cost.TotalCost,