refactor: use explicit dataclasses instead of ORM objects in service layer

Replace dict/ORM object returns with explicit dataclasses to fix
DetachedInstanceError when accessing ORM attributes after session closes.

- Add app/assets/services/schemas.py with AssetData, AssetInfoData,
  AssetDetailResult, and RegisterAssetResult dataclasses
- Update asset_management.py and ingest.py to return dataclasses
- Update manager.py to use attribute access on dataclasses
- Fix created_new to be False in create_asset_from_hash (content exists)
- Add DependencyMissingError for better blake3 missing error handling
- Update tests to use attribute access instead of dict subscripting

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests-unit/assets_test/services/test_asset_management.py

@@ -55,9 +55,9 @@ class TestGetAssetDetail:
         result = get_asset_detail(asset_info_id=info.id)
 
         assert result is not None
-        assert result["info"].id == info.id
-        assert result["asset"].id == asset.id
-        assert set(result["tags"]) == {"alpha", "beta"}
+        assert result.info.id == info.id
+        assert result.asset.hash == asset.hash
+        assert set(result.tags) == {"alpha", "beta"}
 
     def test_respects_owner_visibility(self, mock_create_session, session: Session):
         asset = _make_asset(session)
@@ -102,8 +102,8 @@ class TestUpdateAssetMetadata:
             tags=["new1", "new2"],
         )
 
-        assert set(result["tags"]) == {"new1", "new2"}
-        assert "old" not in result["tags"]
+        assert set(result.tags) == {"new1", "new2"}
+        assert "old" not in result.tags
 
     def test_updates_user_metadata(self, mock_create_session, session: Session):
         asset = _make_asset(session)

tests-unit/assets_test/services/test_ingest.py

@@ -163,8 +163,8 @@ class TestRegisterExistingAsset:
             tags=["models"],
         )
 
-        assert result["created"] is True
-        assert "models" in result["tags"]
+        assert result.created is True
+        assert "models" in result.tags
 
         # Verify by re-fetching from DB
         session.expire_all()
@@ -197,7 +197,7 @@ class TestRegisterExistingAsset:
             owner_id="",
         )
 
-        assert result["created"] is False
+        assert result.created is False
 
         # Verify only one AssetInfo exists for this name
         session.expire_all()
@@ -223,5 +223,5 @@ class TestRegisterExistingAsset:
             tags=["alpha", "beta"],
         )
 
-        assert result["created"] is True
-        assert set(result["tags"]) == {"alpha", "beta"}
+        assert result.created is True
+        assert set(result.tags) == {"alpha", "beta"}