fix(canvas): 画布版本 - 合并 PR #528 防止旧内容覆盖并支持历史恢复

将画布 revision CAS 与历史快照接到 schema v23，保留当前顶栏积分弹窗和已有菜单入口。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/model/models_project.go

@@ -374,6 +374,7 @@ type CanvasProject struct {
 	ProjectID   string    `json:"projectId,omitempty" gorm:"index;size:36;index:idx_canvas_projects_user_project_updated,priority:2"`
 	Title       string    `json:"title" gorm:"size:240"`
 	PayloadJSON string    `json:"payloadJson" gorm:"type:text"`
+	Revision    int64     `json:"revision" gorm:"not null;default:1"`
 	CreatedAt   time.Time `json:"createdAt"`
 	UpdatedAt   time.Time `json:"updatedAt" gorm:"index:idx_canvas_projects_user_updated,priority:2;index:idx_canvas_projects_user_project_updated,priority:3"`
 }