backups: record that a config was checked, not only that it changed

The stale-backup card could not be built as designed, and the reason is more
important than the card. Dedup means an unchanged configuration writes no
revision, so collectedat moves only on a CHANGE. A machine stable for six
months has a six-month-old newest revision and is perfectly healthy. Keying a
staleness card on revision age would have flagged most of the fleet - exactly
the noise that makes a board worth ignoring.

Underneath that: ShopDB could not distinguish those cases at all. On a no-op
the server returned "unchanged" and wrote nothing, so "we checked yesterday and
it matched" was discarded. That fact is the one thing a backup system must be
able to prove, and the only record of it was a line in a log file on the PC.

lastseenat records the check rather than the change. Touched on every matching
post including the no-op; set on creation, since a new revision has by
definition just been seen; backfilled from collectedat or createdat so existing
rows start from the last moment the config can be PROVEN current, rather than
from now - claiming a check that never happened would be worse than silence.

The card keys on it, one row per CHAIN rather than per asset: a machine with
two part markers can have one still reporting while the other stopped, and a
per-asset view would report the machine as fine. It stays deliberately silent
about assets never backed up, because whether one SHOULD be is a question only
the manifest can answer, and guessing would list a hundred healthy machines.

The rule lives in services/staleness.py rather than the route, so it is
testable without an auth layer in the way - the same split retention.py uses.

Threshold is backups_staledays, default 3, and 0 disables the card.

plugins/backups/plugin.py

@@ -161,6 +161,17 @@ class BackupsPlugin(BasePlugin):
                 'description': 'UNC root that opaque (non-JSON) backups are '
                                'written under by the collecting PC. Site-specific.',
             },
+            {
+                'key': 'backups_staledays',
+                'value': '3',
+                'valuetype': 'integer',
+                'category': 'backups',
+                'description': 'Days without a CONFIRMED check before a backup '
+                               'is listed as stopped on the dashboard. Measured '
+                               'from the last check, not the last change - an '
+                               'unchanged config writes no revision. 0 disables '
+                               'the card.',
+            },
             {
                 'key': 'backups_retentioncount',
                 'value': '50',
@@ -180,6 +191,36 @@ class BackupsPlugin(BasePlugin):
             },
         ]
 
+    def get_dashboard_widgets(self) -> List[Dict]:
+        """Dashboard card: backups that have stopped running.
+
+        Keyed on the last CONFIRMED check, never on the last change. Dedup means
+        an unchanged config writes no revision, so a card keyed on revision age
+        would flag most of a healthy fleet.
+        """
+        return [
+            {
+                'id': 'backups-stale',
+                'title': 'Backups stopped',
+                'endpoint': '/api/backups/dashboard/stale',
+                'render': 'exceptions',
+                'severity': 'warning',
+                'permission': 'backups.view',
+                'empty': 'hide',
+                'position': 30,
+                'map': {
+                    'title': 'assetnumber',
+                    'detail': 'backupkind',
+                    'meta': [
+                        {'key': 'sourcehostname', 'label': 'from'},
+                        {'key': 'quietdays', 'label': 'last checked',
+                         'suffix': ' days ago'},
+                    ],
+                    'link': '/backups/asset/{assetid}',
+                },
+            },
+        ]
+
     # ---- ADR-006 collector contract -------------------------------------
 
     def get_collector_schema(self) -> Optional[dict]:
@@ -299,6 +340,13 @@ class BackupsPlugin(BasePlugin):
                   .first())
 
         if latest is not None and latest.contenthash == contenthash:
+            # Unchanged, so no revision - but RECORD THE CHECK. Without this the
+            # only evidence a backup still runs is a line in a log on the PC,
+            # and a config stable for six months is indistinguishable from a
+            # backup that died six months ago. Cheap: one column on a row that
+            # already exists, no new history.
+            latest.lastseenat = datetime.utcnow()
+            db.session.flush()
             return {
                 'action': 'noop',
                 'assetid': assetid,
@@ -330,6 +378,7 @@ class BackupsPlugin(BasePlugin):
             bytesize=bytesize,
             sourcehostname=payload.get('sourcehostname'),
             collectedat=collectedat or datetime.utcnow(),
+            lastseenat=datetime.utcnow(),
         )
         revision.payload = projection
         db.session.add(revision)