fix(monitor): the observation history of a disk is permanent

- An observation is never left out for its age and cannot be dismissed:
  the rule that hid it after 30 days without a repetition, and the
  clean-up that marked the ones of a missing device, are removed.
- The ones earlier versions left out are shown again; an error that came
  back while hidden appears with its first date and its full count.
- The audit attributes recorded events to a disk by its serial number.

AppImage/scripts/flask_server.py

@@ -4620,8 +4620,6 @@ def get_storage_info():
             for disk_name, disk_info in physical_disks.items():
                 disk_info['observations_count'] = health_persistence.count_disk_observations(
                     disk_name, disk_info.get('serial', ''))
-            # Auto-dismiss stale observations (> 30 days old)
-            health_persistence.cleanup_stale_observations()
         except Exception:
             pass