fix: revert ent schema change to fix runtime panic

backend/ent/schema/usage_log.go

@@ -53,9 +53,6 @@ func (UsageLog) Fields() []ent.Field {
 			MaxLen(100).
 			Optional().
 			Nillable(),
-		field.Int64("channel_id").Optional().Nillable().Comment("渠道 ID"),
-		field.String("model_mapping_chain").MaxLen(500).Optional().Nillable().Comment("模型映射链"),
-		field.String("billing_tier").MaxLen(50).Optional().Nillable().Comment("计费层级标签"),
 		field.Int64("group_id").
 			Optional().
 			Nillable(),