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

The plugin shipped five settings and no page for any of them. The core settings
catalog is a hardcoded list a plugin cannot extend, so backups_shareroot and the
collection interval were reachable only through the API or a flask shell on the
server - which is not a configurable setting in any useful sense, and it is how
today went: the setting simply was not there to set.

The page is contributed the ADR-010 way rather than by editing core - a card
from get_settings_cards plus a route in the plugin's own routes.js, both of
which disappear when the plugin is disabled. Filed under Machines, next to
Machine Types, because that is where the assets these back up live.

It also states the two things the settings do not say for themselves: that
health is judged on when a backup was last CONFIRMED rather than when it last
changed, and that retention prunes database revisions only - files on the share
stay, so enabling it on a share-backed kind leaves orphaned snapshot
directories.

Snapshot paths are now shown as full UNC text in both places: a Snapshot row on
the Marker Contents card, and the existing path on the history page, which
gains a copy button. Deliberately NOT a link - browsers refuse to navigate from
an https page to file://, so a clickable path looks actionable and silently
does nothing, which is worse than no link. Copy, then paste into Explorer.

The path comes off the revision, never composed from the configured share root:
a snapshot written before the layout changed still lives where it was put.

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.