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/schema.sql

@@ -195,7 +195,8 @@ CREATE TABLE IF NOT EXISTS canvas_nodes (
     flip_x BOOLEAN NOT NULL DEFAULT FALSE,
     flip_y BOOLEAN NOT NULL DEFAULT FALSE,
     rotation DOUBLE PRECISION NOT NULL DEFAULT 0,
-    semantic_context TEXT NOT NULL DEFAULT ''
+    semantic_context TEXT NOT NULL DEFAULT '',
+    render_content TEXT NOT NULL DEFAULT ''
 );
 
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS user_id UUID;
@@ -227,6 +228,7 @@ ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS flip_x BOOLEAN NOT NULL DEFAUL
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS flip_y BOOLEAN NOT NULL DEFAULT FALSE;
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS rotation DOUBLE PRECISION NOT NULL DEFAULT 0;
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS semantic_context TEXT NOT NULL DEFAULT '';
+ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS render_content TEXT NOT NULL DEFAULT '';
 
 CREATE INDEX IF NOT EXISTS canvas_nodes_project_id_idx ON canvas_nodes(project_id);
 CREATE INDEX IF NOT EXISTS canvas_nodes_user_project_idx ON canvas_nodes(user_id, project_id);