feat(openai): 同步生图 API 支持并接入图片计费调度

- 同步 OpenAI 图片生成与编辑接口
- 接入图片请求解析、账号调度、转发与用量记录
- 接入图片计费与图片用量落库
- 限制 OAuth 生图仅支持无显式模型和尺寸的基础请求

backend/internal/service/model_pricing_resolver.go

@@ -61,6 +61,25 @@ type PricingInput struct {
 // 1. 获取基础定价（LiteLLM → Fallback）
 // 2. 如果指定了 GroupID，查找渠道定价并覆盖
 func (r *ModelPricingResolver) Resolve(ctx context.Context, input PricingInput) *ResolvedPricing {
+	var chPricing *ChannelModelPricing
+	if input.GroupID != nil && r.channelService != nil {
+		chPricing = r.channelService.GetChannelModelPricing(ctx, *input.GroupID, input.Model)
+		if chPricing != nil {
+			mode := chPricing.BillingMode
+			if mode == "" {
+				mode = BillingModeToken
+			}
+			if mode == BillingModePerRequest || mode == BillingModeImage {
+				resolved := &ResolvedPricing{
+					Mode:   mode,
+					Source: PricingSourceChannel,
+				}
+				r.applyRequestTierOverrides(chPricing, resolved)
+				return resolved
+			}
+		}
+	}
+
 	// 1. 获取基础定价
 	basePricing, source := r.resolveBasePricing(input.Model)
 
@@ -72,7 +91,10 @@ func (r *ModelPricingResolver) Resolve(ctx context.Context, input PricingInput)
 	}
 
 	// 2. 如果有 GroupID，尝试渠道覆盖
-	if input.GroupID != nil {
+	if chPricing != nil {
+		resolved.Source = PricingSourceChannel
+		r.applyTokenOverrides(chPricing, resolved)
+	} else if input.GroupID != nil {
 		r.applyChannelOverrides(ctx, *input.GroupID, input.Model, resolved)
 	}