feat(server): persist renderContent texture and cover it in asset lifecycle

Add an optional renderContent field to canvas nodes across the go-zero API
type, domain node, SQLC models/queries, PostgreSQL schema, and mappers so a
lightweight transparent-WebP display texture persists alongside the original
content. Prefer renderContent for project thumbnails and main-canvas
previews, and treat both content and renderContent as protected/deletable
asset URLs during canvas asset cleanup. Extend round-trip coverage.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/logic/query_project_logic.go

@@ -109,7 +109,11 @@ func queryProjectCoverList(project design.Project) []string {
 			break
 		}
 		if (node.Type == design.NodeTypeImage || node.Type == design.NodeTypeFrame) && node.Status == "success" {
-			add(node.Content)
+			if strings.TrimSpace(node.RenderContent) != "" {
+				add(node.RenderContent)
+			} else {
+				add(node.Content)
+			}
 		}
 	}
 	return covers