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/application/design_service.go

@@ -5344,7 +5344,10 @@ func mainCanvasPreviewImage(nodes []design.Node) string {
 		if node.Status == "generating" || node.Status == "error" || node.LayerRole == "pen-stroke" {
 			continue
 		}
-		content := strings.TrimSpace(node.Content)
+		content := strings.TrimSpace(node.RenderContent)
+		if content == "" {
+			content = strings.TrimSpace(node.Content)
+		}
 		if content == "" || strings.HasPrefix(content, "data:image/") || !isGeneratedImageContent(content) {
 			continue
 		}