fix(*): 删除闭环 - 统一服务端真相并释放画布素材关联

backend/internal/repository/repository.go

@@ -999,7 +999,22 @@ func (r *Repository) UpsertCanvasProject(project *model.CanvasProject) error {
 }
 
 func (r *Repository) DeleteCanvasProject(userID string, id string) error {
-	return r.db.Delete(&model.CanvasProject{}, "id = ? AND user_id = ?", id, userID).Error
+	return r.db.Transaction(func(tx *gorm.DB) error {
+		if err := tx.Where("user_id = ? AND project_id = ?", userID, id).Delete(&model.CanvasShare{}).Error; err != nil {
+			return err
+		}
+		if err := tx.Where("canvas_id = ?", id).Delete(&model.CanvasUnitLink{}).Error; err != nil {
+			return err
+		}
+		// 任务和会话是审计记录，不随独立画布实体保留归属 ID，避免删除后继续挂住画布上下文。
+		if err := tx.Model(&model.Task{}).Where("user_id = ? AND project_id = ?", userID, id).Update("project_id", "").Error; err != nil {
+			return err
+		}
+		if err := tx.Model(&model.Session{}).Where("user_id = ? AND project_id = ?", userID, id).Update("project_id", "").Error; err != nil {
+			return err
+		}
+		return tx.Delete(&model.CanvasProject{}, "id = ? AND user_id = ?", id, userID).Error
+	})
 }
 
 func (r *Repository) Projects(userID string) ([]model.Project, error) {