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>

app/assets/api/routes.py

@@ -12,6 +12,7 @@ from app.assets.api import schemas_in
 from app.assets.api.schemas_in import (
     AssetNotFoundError,
     AssetValidationError,
+    DependencyMissingError,
     HashMismatchError,
     UploadError,
 )
@@ -205,6 +206,8 @@ async def upload_asset(request: web.Request) -> web.Response:
         return _build_error_response(404, "ASSET_NOT_FOUND", str(e))
     except HashMismatchError as e:
         return _build_error_response(400, "HASH_MISMATCH", str(e))
+    except DependencyMissingError as e:
+        return _build_error_response(503, "DEPENDENCY_MISSING", e.message)
     except Exception:
         logging.exception("process_upload failed for owner_id=%s", owner_id)
         return _build_error_response(500, "INTERNAL", "Unexpected server error.")

app/assets/api/schemas_in.py

@@ -43,6 +43,14 @@ class HashMismatchError(Exception):
     pass
 
 
+class DependencyMissingError(Exception):
+    """A required dependency is not installed."""
+
+    def __init__(self, message: str):
+        super().__init__(message)
+        self.message = message
+
+
 @dataclass
 class ParsedUpload:
     """Result of parsing a multipart upload request."""