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,
 	}

backend/internal/handler/dto/mappers.go

@@ -577,6 +577,7 @@ func usageLogFromServiceUser(l *service.UsageLog) UsageLog {
 		MediaType:             l.MediaType,
 		UserAgent:             l.UserAgent,
 		CacheTTLOverridden:    l.CacheTTLOverridden,
+		BillingMode:           l.BillingMode,
 		CreatedAt:             l.CreatedAt,
 		User:                  UserFromServiceShallow(l.User),
 		APIKey:                APIKeyFromService(l.APIKey),

backend/internal/handler/dto/types.go

@@ -390,6 +390,9 @@ type UsageLog struct {
 	// Cache TTL Override 标记
 	CacheTTLOverridden bool `json:"cache_ttl_overridden"`
 
+	// BillingMode 计费模式：token/image
+	BillingMode *string `json:"billing_mode,omitempty"`
+
 	CreatedAt time.Time `json:"created_at"`
 
 	User         *User             `json:"user,omitempty"`