backups: show that a check happened, not just that a change did

lastseenat already recorded it and the API already returned it; nothing
displayed it, so from the UI a healthy machine still looked abandoned - one
revision from last spring and no sign anything had looked at it since.

The history page gains a Last verified column beside Captured, and the asset
panel a Verified field. Only the CURRENT revision carries one: an older
revision was superseded, so saying it was verified today would be false - what
was verified is the configuration the PC holds now.

A current revision with no check yet says "not yet checked" rather than showing
a blank or borrowing the captured date. That state is real and temporary: the
column is new, so every chain reports it until its PC next posts.

CHANGELOG.md

@@ -12,6 +12,13 @@ ADR-007 and ADR-002.
 
 ### Added
 
+- Backup history distinguishes CAPTURED from LAST VERIFIED. Captured is when a
+  configuration was written; verified is when a PC last posted it and the hash
+  still matched. They differ by design, because an unchanged config writes no
+  new revision - so a machine checked daily for a year shows one revision from
+  last spring, and previously read as abandoned. Shown on the history page and
+  on the asset panel; only the current revision carries it, since an older one
+  is not what the PC holds now.
 - A backup now records that it was CHECKED, not only that it changed. Dedup
   means an unchanged configuration writes no revision, so the stored timestamps
   moved only on a change: a machine whose settings had been stable for six