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_portfolio_service.py

@@ -268,7 +268,7 @@ async def test_fetch_historical_prices_skips_none(mock_fetch_hist):
 # ---------------------------------------------------------------------------
 
 
-def _make_prices(symbols: list[str], n_days: int = 60) -> "pd.DataFrame":
+def _make_prices(symbols: list[str], n_days: int = 60):
     """Build a deterministic price DataFrame with enough rows for t-SNE."""
     import numpy as np
     import pandas as pd
@@ -383,7 +383,6 @@ async def test_cluster_stocks_no_data_raises(mock_fetch):
 @patch("portfolio_service.fetch_historical_prices", new_callable=AsyncMock)
 async def test_cluster_stocks_identical_returns_still_works(mock_fetch):
     """t-SNE should not raise even when all symbols have identical returns."""
-    import numpy as np
     import pandas as pd
     import portfolio_service
 
@@ -530,7 +529,6 @@ async def test_find_similar_stocks_no_data_raises(mock_fetch):
 @patch("portfolio_service.fetch_historical_prices", new_callable=AsyncMock)
 async def test_find_similar_stocks_target_not_in_data_raises(mock_fetch):
     """find_similar_stocks raises ValueError when target symbol has no data."""
-    import pandas as pd
     import portfolio_service
 
     # Only universe symbols have data, not the target