fix(node_health): say 'active via <unit>', not a bare parenthetical

The unit name in the swap-active detail read as an unlabelled aside. Naming it as
the unit that activated the swap is what makes the line actionable.

Co-Authored-By: Claude Opus 5 <[email protected]>

infra/ansible/roles/node_health/tasks/assess.yml

@@ -554,7 +554,7 @@
     swap_detail: >-
       {{ 'swap state unreadable' if m.swap_total_kb is not number
          else ((swap_active_gib ~ ' GiB swap active'
-                ~ (' (' ~ swap_units_returning ~ ')' if swap_units_returning else '')
+                ~ (' via ' ~ swap_units_returning if swap_units_returning else '')
                 ~ ' — ' ~ swap_cause)
                if (m.swap_total_kb | int) > 0
                else ('no swap active, but it would come back at the next boot via '