feat(canvas): 条件读取与历史 - 减少画布同步传输并校准版本

backend/internal/repository/canvas_project_read.go

@@ -0,0 +1,15 @@
+package repository
+
+import "infinite-canvas/backend/internal/model"
+
+// CanvasProjectMetadataForUser reads only fields needed to validate a cached
+// canvas representation, avoiding the large JSON body when the caller's ETag
+// is still current.
+func (r *Repository) CanvasProjectMetadataForUser(userID string, id string) (*model.CanvasProject, error) {
+	var project model.CanvasProject
+	if err := r.db.Select("id", "user_id", "project_id", "title", "revision", "created_at", "updated_at").
+		First(&project, "id = ? AND user_id = ?", id, userID).Error; err != nil {
+		return nil, err
+	}
+	return &project, nil
+}