Add copyright header to source code files

Copyright notice as per
https://reuse.software/spec/#format-of-copyright-notices

Fixes #134

src/backend/models/__init__.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 """Provide classes related to data models shared between Python and QML."""

src/backend/models/filters.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 from typing import (

src/backend/models/items.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 """`ModelItem` subclasses definitions."""

src/backend/models/model.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 import itertools

src/backend/models/model_item.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 from dataclasses import dataclass, field

src/backend/models/model_store.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 from collections import UserDict

src/backend/models/proxy.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 from typing import TYPE_CHECKING, Any, Dict, Optional

src/backend/models/special_models.py

@@ -1,3 +1,4 @@
+# Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
 from dataclasses import asdict