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/domain/design/project.go

@@ -130,6 +130,7 @@ type Node struct {
 	Width               float64
 	Height              float64
 	Content             string
+	RenderContent       string
 	Tone                string
 	Status              string
 	ParentID            string