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/project_thumbnail_test.go

@@ -23,13 +23,14 @@ func TestProjectThumbnailUsesCanvasPreview(t *testing.T) {
 				Status:  "success",
 			},
 			{
-				ID:      "main",
-				Type:    design.NodeTypeImage,
-				Title:   "Main",
-				Content: "http://localhost:19000/canvas-assets/main.png",
-				Width:   1024,
-				Height:  768,
-				Status:  "success",
+				ID:            "main",
+				Type:          design.NodeTypeImage,
+				Title:         "Main",
+				Content:       "http://localhost:19000/canvas-assets/main.svg",
+				RenderContent: "http://localhost:19000/canvas-assets/main.render.webp",
+				Width:         1024,
+				Height:        768,
+				Status:        "success",
 			},
 		},
 	}
@@ -49,7 +50,7 @@ func TestProjectThumbnailUsesCanvasPreview(t *testing.T) {
 	if len(preview.Nodes) != 2 {
 		t.Fatalf("expected two preview nodes, got %#v", preview.Nodes)
 	}
-	if preview.Nodes[1].Content != "http://localhost:19000/canvas-assets/main.png" {
+	if preview.Nodes[1].Content != "http://localhost:19000/canvas-assets/main.svg" || preview.Nodes[1].RenderContent != "http://localhost:19000/canvas-assets/main.render.webp" {
 		t.Fatalf("expected generated image in canvas preview, got %#v", preview.Nodes[1])
 	}
 }