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/application/asset_cleanup_test.go

@@ -63,7 +63,7 @@ func TestDeleteProjectEnqueuesAssetCleanupJobWhenJobQueueIsConfigured(t *testing
 		Status:    design.StatusReady,
 		UpdatedAt: time.Now(),
 		Nodes: []design.Node{
-			{ID: "image-1", Type: design.NodeTypeImage, Content: "http://localhost:19000/canvas-assets/a.png", Status: "success"},
+			{ID: "image-1", Type: design.NodeTypeImage, Content: "http://localhost:19000/canvas-assets/a.svg", RenderContent: "http://localhost:19000/canvas-assets/a.render.webp", Status: "success"},
 		},
 	}
 	if err := repo.Save(ctx, project); err != nil {
@@ -78,7 +78,7 @@ func TestDeleteProjectEnqueuesAssetCleanupJobWhenJobQueueIsConfigured(t *testing
 		t.Fatalf("expected one cleanup job, got %#v", queue.jobs)
 	}
 	job := queue.jobs[0]
-	if job.Kind != design.JobAssetCleanup || len(job.PublicURLs) != 1 || job.PublicURLs[0] != "http://localhost:19000/canvas-assets/a.png" {
+	if job.Kind != design.JobAssetCleanup || len(job.PublicURLs) != 2 || job.PublicURLs[0] != "http://localhost:19000/canvas-assets/a.svg" || job.PublicURLs[1] != "http://localhost:19000/canvas-assets/a.render.webp" {
 		t.Fatalf("unexpected cleanup job: %#v", job)
 	}
 	select {