feat(usage): 使用记录增加计费模式字段 — 记录/展示/筛选 token/按次/图片

- DB: usage_logs 表新增 billing_mode VARCHAR(20) 列
- 后端: RecordUsage 写入时根据 image_count 判定计费模式
- 前端: 使用记录表格新增计费模式 badge 列 + 筛选下拉

backend/internal/service/gateway_service.go

@@ -7863,6 +7863,17 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 		CreatedAt:             time.Now(),
 	}
 
+	// 设置计费模式
+	if result.MediaType != "image" && result.MediaType != "video" && result.MediaType != "prompt" {
+		if result.ImageCount > 0 {
+			billingMode := "image"
+			usageLog.BillingMode = &billingMode
+		} else {
+			billingMode := "token"
+			usageLog.BillingMode = &billingMode
+		}
+	}
+
 	// 添加 UserAgent
 	if input.UserAgent != "" {
 		usageLog.UserAgent = &input.UserAgent
@@ -8073,6 +8084,15 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 		CreatedAt:             time.Now(),
 	}
 
+	// 设置计费模式
+	if result.ImageCount > 0 {
+		billingMode := "image"
+		usageLog.BillingMode = &billingMode
+	} else {
+		billingMode := "token"
+		usageLog.BillingMode = &billingMode
+	}
+
 	// 添加 UserAgent
 	if input.UserAgent != "" {
 		usageLog.UserAgent = &input.UserAgent