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/pricing_service.go

@@ -70,7 +70,8 @@ type LiteLLMModelPricing struct {
 	LiteLLMProvider                     string  `json:"litellm_provider"`
 	Mode                                string  `json:"mode"`
 	SupportsPromptCaching               bool    `json:"supports_prompt_caching"`
-	OutputCostPerImage                  float64 `json:"output_cost_per_image"` // 图片生成模型每张图片价格
+	OutputCostPerImage                  float64 `json:"output_cost_per_image"`       // 图片生成模型每张图片价格
+	OutputCostPerImageToken             float64 `json:"output_cost_per_image_token"` // 图片输出 token 价格
 }
 
 // PricingRemoteClient 远程价格数据获取接口
@@ -94,6 +95,7 @@ type LiteLLMRawEntry struct {
 	Mode                                string   `json:"mode"`
 	SupportsPromptCaching               bool     `json:"supports_prompt_caching"`
 	OutputCostPerImage                  *float64 `json:"output_cost_per_image"`
+	OutputCostPerImageToken             *float64 `json:"output_cost_per_image_token"`
 }
 
 // PricingService 动态价格服务
@@ -408,6 +410,9 @@ func (s *PricingService) parsePricingData(body []byte) (map[string]*LiteLLMModel
 		if entry.OutputCostPerImage != nil {
 			pricing.OutputCostPerImage = *entry.OutputCostPerImage
 		}
+		if entry.OutputCostPerImageToken != nil {
+			pricing.OutputCostPerImageToken = *entry.OutputCostPerImageToken
+		}
 
 		result[modelName] = pricing
 	}