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/db/models.go

@@ -105,6 +105,7 @@ type CanvasNode struct {
 	FlipY           bool        `json:"flip_y"`
 	Rotation        float64     `json:"rotation"`
 	SemanticContext string      `json:"semantic_context"`
+	RenderContent   string      `json:"render_content"`
 }
 
 type CanvasSnapshot struct {