fix(flow): 发布 backthink 会话的 graph_update——后台思考页链路导图不再是'等待事件'

FlowPublisher allow 谓词只允许 web/web:*，sess-backthink 被排除→backthink 的
输入→思考→输出图从不发布，后台思考页'查看链路导图'恒为'等待 graph_update 事件'。
- backthink.go: 导出 DefaultSessionID="sess-backthink"
- wire.go: 谓词追加 sessionID==backthink.DefaultSessionID（web + backthink 都发；sess-xxx 隔离仍过滤）
go build/vet/test 全绿; 已重建部署. 下次 backthink(5min心跳)运行时即发布该会话图.

internal/services/backthink/backthink.go

@@ -64,6 +64,11 @@ const (
 	defaultSessionID = "sess-backthink"
 )
 
+// DefaultSessionID is the exported loop.Run session key for an autonomous think. It is
+// exposed so the flow-viz publisher (cmd/cyrene) can also publish the backthink flow
+// graph, which the backthink page and the web-chat flow page both render.
+const DefaultSessionID = "sess-backthink"
+
 // Sentinel errors returned by this package. Use errors.Is to test for them; they
 // are returned directly or wrapped with %w so the underlying cause stays intact.
 var (