Audit updates last_audit but not last_seen when propertiesChanged = False

[mbr-g]

Issue description:
When the audit function in runners/inventory.py runs without any changes in minion data, it updates last_audit but not last_seen. Since a successful audit implies that a minion was reachable, it would make more sense for last_seen to be updated as well.

Currently, this leads to inconsistent states where an audit appears to have occurred before the minion was last seen. Because last_seen is used as an availability metric, updating it alongside last_audit would provide a more accurate reflection of minion activity.

The purpose of last_audit is also not entirely clear. It is not obvious to the end user whether it indicates that a minion has new data, or simply that the audit ran successfully. Updating last_seen may help clarify this distinction, otherwise, a mention of this in the documentation or a UI tooltip would make it more obvious.

[neilmunday]

The last_seen value is updated when presence events are registered by minions. See https://docs.saltproject.io/en/latest/topics/event/master_events.html#presence-events for further details.

These events are useful to detect when a minion last connected to the Salt-Master. If the minion has not been seen before, then this also trigger a one-off audit to populate the database. In this instance the last_seen and last_audit values will be roughly equal.

Outside of this, audit events occur every 6 hours and updates details about the minion in the database.

[neilmunday]

I have updated the README to include additional information. Please see: https://github.com/neilmunday/Salt-Minion-Inventory?tab=readme-ov-file#last-seen-vs-last-audit