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/project_thumbnail.go

@@ -31,6 +31,7 @@ type projectPreviewNode struct {
 	Width          float64 `json:"width"`
 	Height         float64 `json:"height"`
 	Content        string  `json:"content,omitempty"`
+	RenderContent  string  `json:"renderContent,omitempty"`
 	Tone           string  `json:"tone,omitempty"`
 	LayerRole      string  `json:"layerRole,omitempty"`
 	FillColor      string  `json:"fillColor,omitempty"`
@@ -82,6 +83,7 @@ func canvasPreviewNodes(nodes []design.Node) []projectPreviewNode {
 			continue
 		}
 		content := strings.TrimSpace(node.Content)
+		renderContent := strings.TrimSpace(node.RenderContent)
 		if node.Type == design.NodeTypeFrame && !isGeneratedImageContent(content) {
 			content = ""
 		}
@@ -93,6 +95,7 @@ func canvasPreviewNodes(nodes []design.Node) []projectPreviewNode {
 			Width:          math.Max(1, node.Width),
 			Height:         math.Max(1, node.Height),
 			Content:        content,
+			RenderContent:  renderContent,
 			Tone:           strings.TrimSpace(node.Tone),
 			LayerRole:      strings.TrimSpace(node.LayerRole),
 			FillColor:      strings.TrimSpace(node.FillColor),