chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/tests/test_task_manager.py

@@ -7,12 +7,14 @@
 
 import sys
 from pathlib import Path
+
 sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
 
-import pytest
 import asyncio
-from unittest.mock import MagicMock, patch, AsyncMock
 from datetime import datetime
+from unittest.mock import MagicMock, patch
+
+import pytest
 
 
 # Helper to create a TaskManager with mocked dependencies