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/cad_context_mapper_test.go

@@ -13,10 +13,11 @@ func TestCanvasNodeSemanticContextRoundTrip(t *testing.T) {
 		Title:           "plan.svg",
 		LayerRole:       "cad-vector",
 		SemanticContext: "CAD semantic context: layer ROOMS",
+		RenderContent:   "https://assets.example/plan-preview.webp",
 	}}
 
 	roundTrip := toDomainNodes(toAPINodes(nodes))
-	if len(roundTrip) != 1 || roundTrip[0].SemanticContext != nodes[0].SemanticContext {
-		t.Fatalf("expected semantic context round trip, got %#v", roundTrip)
+	if len(roundTrip) != 1 || roundTrip[0].SemanticContext != nodes[0].SemanticContext || roundTrip[0].RenderContent != nodes[0].RenderContent {
+		t.Fatalf("expected CAD context and render content round trip, got %#v", roundTrip)
 	}
 }

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,

server/internal/logic/query_project_logic.go

@@ -109,7 +109,11 @@ func queryProjectCoverList(project design.Project) []string {
 			break
 		}
 		if (node.Type == design.NodeTypeImage || node.Type == design.NodeTypeFrame) && node.Status == "success" {
-			add(node.Content)
+			if strings.TrimSpace(node.RenderContent) != "" {
+				add(node.RenderContent)
+			} else {
+				add(node.Content)
+			}
 		}
 	}
 	return covers