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/test_disk_observation.py

@@ -103,7 +103,6 @@ def main():
             print(f"    First: {obs['first_occurrence']}")
             print(f"    Last: {obs['last_occurrence']}")
             print(f"    Count: {obs['occurrence_count']}")
-            print(f"    Dismissed: {obs['dismissed']}")
             print()
     else:
         print("    No observations found!\n")