refactor(assets): modular architecture + async two-phase scanner & background seeder (#12621)

comfy_api/feature_flags.py

@@ -15,6 +15,7 @@ SERVER_FEATURE_FLAGS: dict[str, Any] = {
     "max_upload_size": args.max_upload_size * 1024 * 1024, # Convert MB to bytes
     "extension": {"manager": {"supports_v4": True}},
     "node_replacements": True,
+    "assets": args.enable_assets,
 }