Controller should allow grouping units by tag to target batch operations

[cotosso]

Description

The controller manages units as a flat list, with no way to organise them by criteria such as location, customer or update policy.
Since there is no persistent grouping to target, any action on a subset of the installed base requires selecting the units individually every time.

Expected Behavior

• Tags can be assigned to units in the controller.
• Units can be selected by tag, for example to group machines that are geographically close.
• A batch update can be scheduled on the units selected by a tag