Add SPDX ID LGPL v3+ license headers

src/backend/models/__init__.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+
 """Provide classes related to data models shared between Python and QML."""
 
 from typing import Tuple, Type, Union

src/backend/models/items.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+
 """`ModelItem` subclasses definitions."""
 
 import asyncio

src/backend/models/model.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+
 import logging as log
 import time
 from threading import Lock, Thread

src/backend/models/model_item.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+
 from typing import Any, Dict, Optional
 
 from ..utils import serialize_value_for_qml

src/backend/models/model_store.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+
 from collections import UserDict
 from dataclasses import dataclass, field
 from typing import Dict