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/mapper.go

@@ -259,6 +259,7 @@ func toAPINodes(items []design.Node) []types.CanvasNode {
 			Width:               item.Width,
 			Height:              item.Height,
 			Content:             item.Content,
+			RenderContent:       item.RenderContent,
 			Tone:                item.Tone,
 			Status:              item.Status,
 			ParentId:            item.ParentID,
@@ -302,6 +303,7 @@ func toDomainNodes(items []types.CanvasNode) []design.Node {
 			Width:               item.Width,
 			Height:              item.Height,
 			Content:             item.Content,
+			RenderContent:       item.RenderContent,
 			Tone:                item.Tone,
 			Status:              item.Status,
 			ParentID:            item.ParentId,