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/handler/admin/usage_handler.go

@@ -110,6 +110,7 @@ func (h *UsageHandler) List(c *gin.Context) {
 	}
 
 	model := c.Query("model")
+	billingMode := strings.TrimSpace(c.Query("billing_mode"))
 
 	var requestType *int16
 	var stream *bool
@@ -174,6 +175,7 @@ func (h *UsageHandler) List(c *gin.Context) {
 		RequestType: requestType,
 		Stream:      stream,
 		BillingType: billingType,
+		BillingMode: billingMode,
 		StartTime:   startTime,
 		EndTime:     endTime,
 		ExactTotal:  exactTotal,
@@ -234,6 +236,7 @@ func (h *UsageHandler) Stats(c *gin.Context) {
 	}
 
 	model := c.Query("model")
+	billingMode := strings.TrimSpace(c.Query("billing_mode"))
 
 	var requestType *int16
 	var stream *bool
@@ -312,6 +315,7 @@ func (h *UsageHandler) Stats(c *gin.Context) {
 		RequestType: requestType,
 		Stream:      stream,
 		BillingType: billingType,
+		BillingMode: billingMode,
 		StartTime:   &startTime,
 		EndTime:     &endTime,
 	}