fix: 自动标题取消语义、清洗循环与拒绝占位回写

spawn_task 标题 20s 超时且父 ctx 取消不建任务；清洗循环剥前缀内外引号；
异步标题拒绝「新对话」并用条件更新防手改竞态；会话 profile 失败回退全局。

db/conversation.go

@@ -197,6 +197,15 @@ func (d *DB) RenameConversation(id int64, title string) error {
 	return err
 }
 
+// RenameConversationIfTitle updates a thread only while its title is still an
+// initial placeholder or preview. This keeps asynchronous auto-title writes
+// from overwriting a concurrent manual rename.
+func (d *DB) RenameConversationIfTitle(id int64, title, initial string) error {
+	_, err := d.Exec(`UPDATE conversations SET title=$2
+WHERE id=$1 AND title IN ('', '新对话', $3)`, id, title, initial)
+	return err
+}
+
 // TouchConversation bumps updated_at so the thread floats to the top of the list.
 func (d *DB) TouchConversation(id int64) error {
 	_, err := d.Exec(`UPDATE conversations SET updated_at=now() WHERE id=$1`, id)