chore: fix all ruff lint warnings

- Remove unused datetime imports from openbb_service, market_service,
  quantitative_service (now using obb_utils.days_ago)
- Remove unused variable 'maintains' in routes_sentiment
- Remove unused imports in test files
- Fix forward reference annotation in test helper

tests/test_akshare_service.py

@@ -1,7 +1,6 @@
 """Unit tests for akshare_service.py - written FIRST (TDD RED phase)."""
 
-from datetime import datetime
-from unittest.mock import MagicMock, patch
+from unittest.mock import patch
 
 import pandas as pd
 import pytest