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revision was superseded, so saying it was verified today would be false - what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.

plugins/backups/frontend/views/BackupHistory.vue

@@ -20,6 +20,7 @@
       <thead>
         <tr>
           <th>Captured</th>
+          <th>Last verified</th>
           <th>Kind</th>
           <th>From PC</th>
           <th>Hash</th>
@@ -34,6 +35,22 @@
             {{ formatWhen(rev.collectedat || rev.createdat) }}
             <span v-if="rev.islatest" class="bh-badge">current</span>
           </td>
+          <!-- CAPTURED is when this configuration was written; LAST VERIFIED is
+               when a PC last posted it and the hash still matched. They differ
+               by design: an unchanged config writes no new revision, so a
+               machine can be checked daily for a year and still show one
+               revision from last spring. Without this column that machine
+               looked abandoned. Only the current revision is verified - an
+               older one was superseded, so it is not what the PC holds now. -->
+          <td>
+            <template v-if="rev.islatest && rev.lastseenat">
+              {{ formatWhen(rev.lastseenat) }}
+            </template>
+            <span v-else-if="rev.islatest" class="bh-muted"
+                  title="No check recorded since this was introduced. The next
+                         collection will record one.">not yet checked</span>
+            <span v-else class="bh-muted">-</span>
+          </td>
           <td>{{ rev.backupkind }}</td>
           <td>{{ rev.sourcehostname || '-' }}</td>
           <td class="bh-mono">{{ rev.shorthash }}</td>

plugins/backups/plugin.py

@@ -101,6 +101,12 @@ class BackupsPlugin(BasePlugin):
                     'title': 'label',
                     'meta': [
                         {'key': 'collectedat', 'label': 'Captured', 'format': 'date'},
+                        # Captured is when this config was WRITTEN; verified is
+                        # when a PC last posted it and the hash still matched.
+                        # An unchanged config writes no revision, so without
+                        # this a machine checked daily for a year shows one
+                        # entry from last spring and reads as abandoned.
+                        {'key': 'lastseenat', 'label': 'Verified', 'format': 'date'},
                         {'key': 'sourcehostname', 'label': 'From'},
                         {'key': 'shorthash', 'label': 'Hash', 'mono': True},
                     ],