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/infrastructure/postgres/project_repository.go

@@ -199,6 +199,7 @@ func (r *ProjectRepository) Save(ctx context.Context, project design.Project) er
 			Width:           node.Width,
 			Height:          node.Height,
 			Content:         node.Content,
+			RenderContent:   node.RenderContent,
 			Tone:            node.Tone,
 			Status:          node.Status,
 			SortOrder:       int32(index),
@@ -518,6 +519,7 @@ func mapNodes(rows []sqlcdb.CanvasNode) []design.Node {
 			Width:           row.Width,
 			Height:          row.Height,
 			Content:         row.Content,
+			RenderContent:   row.RenderContent,
 			Tone:            row.Tone,
 			Status:          row.Status,
 			ParentID:        row.ParentID,