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/types/types.go

@@ -283,6 +283,7 @@ type CanvasNode struct {
 	Width               float64       `json:"width"`
 	Height              float64       `json:"height"`
 	Content             string        `json:"content"`
+	RenderContent       string        `json:"renderContent,optional"`
 	Tone                string        `json:"tone"`
 	Status              string        `json:"status"`
 	ParentId            string        `json:"parentId,optional"`