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/delete_running_tasks.py

@@ -7,9 +7,11 @@
 # @RELATION: DEPENDS_ON ->[TaskRecord]
 
 from sqlalchemy.orm import Session
+
 from src.core.database import TasksSessionLocal
 from src.models.task import TaskRecord
 
+
 # [DEF:delete_running_tasks:Function]
 # @PURPOSE: Delete all tasks with RUNNING status from the database.
 # @PRE: Database is accessible and TaskRecord model is defined.
@@ -20,19 +22,19 @@ def delete_running_tasks():
     try:
         # Find all task records with RUNNING status
         running_tasks = session.query(TaskRecord).filter(TaskRecord.status == "RUNNING").all()
-        
+
         if not running_tasks:
             print("No RUNNING tasks found.")
             return
-        
+
         print(f"Found {len(running_tasks)} RUNNING tasks:")
         for task in running_tasks:
             print(f"- Task ID: {task.id}, Type: {task.type}")
-        
+
         # Delete the found tasks
         session.query(TaskRecord).filter(TaskRecord.status == "RUNNING").delete(synchronize_session=False)
         session.commit()
-        
+
         print(f"Successfully deleted {len(running_tasks)} RUNNING tasks.")
     except Exception as e:
         session.rollback()