feat: improve civitai data handling and type safety, fixes #565

- Replace setdefault with get and explicit dict initialization in MetadataUpdater
- Change civitai field type from Optional[Dict] to Dict[str, Any] with default_factory
- Add None check and dict initialization in BaseModelMetadata.__post_init__
- Ensures civitai data is always a dictionary, preventing type errors and improving code reliability

tests/utils/test_metadata_defaults.py

@@ -0,0 +1,37 @@
+import json
+import pytest
+
+from py.utils.metadata_manager import MetadataManager
+from py.utils.models import BaseModelMetadata
+
+
+@pytest.mark.asyncio
+async def test_base_model_metadata_sets_empty_civitai_dict():
+    metadata = BaseModelMetadata(
+        file_name="model",
+        model_name="Model",
+        file_path="/tmp/model.safetensors",
+        size=0,
+        modified=0.0,
+        sha256="deadbeef",
+        base_model="Unknown",
+        preview_url="",
+    )
+
+    assert metadata.civitai == {}
+
+
+@pytest.mark.asyncio
+async def test_create_default_metadata_uses_empty_civitai(tmp_path):
+    model_path = tmp_path / "example.safetensors"
+    model_path.write_bytes(b"stub")
+
+    metadata = await MetadataManager.create_default_metadata(str(model_path))
+
+    assert metadata is not None
+    assert metadata.civitai == {}
+
+    metadata_path = model_path.with_suffix(".metadata.json")
+    payload = json.loads(metadata_path.read_text(encoding="utf-8"))
+
+    assert payload.get("civitai") == {}