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_persistence.py

@@ -6,16 +6,17 @@
 # @TEST_DATA: valid_task -> {"id": "test-uuid-1", "plugin_id": "backup", "status": "PENDING"}
 
 # [SECTION: IMPORTS]
-from datetime import datetime, timedelta
+from datetime import datetime
 from unittest.mock import patch
 
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker
 
+from src.core.task_manager.models import LogEntry, Task, TaskStatus
+from src.core.task_manager.persistence import TaskPersistenceService
 from src.models.mapping import Base, Environment
 from src.models.task import TaskRecord
-from src.core.task_manager.persistence import TaskPersistenceService
-from src.core.task_manager.models import Task, TaskStatus, LogEntry
+
 # [/SECTION]