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/api/routes.py

@@ -344,3 +344,19 @@ def _diffprojections(old, new):
             'aftertype': None if after is None else after[0],
         })
     return changes
+
+
+@backups_bp.route('/dashboard/stale', methods=['GET'])
+@jwt_required()
+@require_permission('backups.view')
+def dashboard_stale():
+    """Chains whose backup has stopped running.
+
+    Thin: the rule lives in services/staleness.py, where it is testable without
+    an auth layer in the way. 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.
+    """
+    from ..services.staleness import stalechains
+
+    return success_response(stalechains())