refactor: fix linting issues and code quality

- Fix import ordering in __init__.py
- Remove unused imports from dependencies.py
- Fix import sorting across multiple files
- Apply ruff auto-fixes

No functional changes

tests/unit/test_services/test_notebook_service.py

@@ -4,8 +4,8 @@ TDD Cycle: RED → GREEN → REFACTOR
 """
 
 from datetime import datetime
-from unittest.mock import AsyncMock, MagicMock, patch
-from uuid import UUID, uuid4
+from unittest.mock import AsyncMock, MagicMock
+from uuid import uuid4
 
 import pytest