Give the backups plugin a settings page, and show where a snapshot is

plugins/backups/plugin.py

@@ -133,6 +133,31 @@ class BackupsPlugin(BasePlugin):
             panels.append(panel)
         return panels
 
+    def get_settings_cards(self) -> List[Dict]:
+        """ADR-010: contribute the Backups settings card.
+
+        The core settings catalog (settingsNav.js) is a hardcoded list a plugin
+        cannot extend, so this plugin shipped five settings and no page for any
+        of them - backups_shareroot and the collection interval were reachable
+        only through the API or a shell on the server, which is not a
+        configurable setting in any useful sense.
+
+        Filed under Machines because that is where the assets these back up
+        live, and it puts the card next to Machine Types rather than in a
+        platform group nobody visits.
+        """
+        return [
+            {
+                'group': 'Machines',
+                'to': '/settings/backups',
+                'icon': 'package',
+                'title': 'Backups',
+                'description': 'Collection interval, share root for part-marker '
+                               'snapshots, staleness threshold and revision retention',
+                'position': 21,
+            },
+        ]
+
     def get_settings_defaults(self) -> List[Dict]:
         """Schedule + retention, read by the collector and the prune job.