fix(channel): use upstream model for account stats pricing and remove channel pricing fallback

- resolveAccountStatsCost now uses the final upstream model (after
  account-level mapping) to match custom pricing rules, fixing the
  issue where requested model (e.g. claude-sonnet-4-5) didn't match
  rules configured for upstream model (e.g. claude-opus-4-6)
- Remove tryChannelPricing fallback — only custom rules are applied,
  unmatched requests use default formula (total_cost × rate)
- Remove unused billingService and serviceTier parameters
- Update description: "启用后将支持自定义账号统计的模型价格"

backend/internal/service/openai_gateway_service.go

@@ -4573,12 +4573,16 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		usageLog.SubscriptionID = &subscription.ID
 	}
 
-	// 计算账号统计定价费用
+	// 计算账号统计定价费用（使用最终上游模型匹配自定义规则）
 	if apiKey.GroupID != nil {
+		statsModel := result.UpstreamModel
+		if statsModel == "" {
+			statsModel = result.Model
+		}
 		usageLog.AccountStatsCost = resolveAccountStatsCost(
-			ctx, s.channelService, s.billingService,
-			account.ID, *apiKey.GroupID, billingModel,
-			tokens, 1, serviceTier,
+			ctx, s.channelService,
+			account.ID, *apiKey.GroupID, statsModel,
+			tokens, 1,
 		)
 	}