Add job cancellation handling and periodic cleanup logic

Introduce new statuses, "to_cancel" and "cancelled", to improve job state tracking. Implement logic to nullify `slurm_id` for cancelled jobs and a background thread to clean up cancelled jobs older than 2 hours. Ensure periodic cleanup runs hourly to maintain database hygiene.

backend/app/models.py

@@ -313,6 +313,8 @@ class JobStatus(str, enum.Enum):
     TODO = "todo"
     SUBMITTED = "submitted"
     DONE = "done"
+    TO_CANCEL = "to_cancel"
+    CANCELLED = "cancelled"
     FAILED = "failed"