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/api/routes.py

@@ -179,7 +179,11 @@ def asset_info(assetid):
         # The card names the PC a snapshot came from, and chains its diff on the
         # same value. Passed from the column rather than read out of the stored
         # manifest, which carries its own copy that can disagree.
-        sourcehostname=revision.sourcehostname)
+        sourcehostname=revision.sourcehostname,
+        # Where the snapshot actually is. Read off the revision rather than
+        # composed from the configured share root, so a snapshot written before
+        # the layout changed still points at where it was really put.
+        sharepath=revision.sharepath)
     data['backuprevisionid'] = revision.backuprevisionid
     data['collectedat'] = _utciso(revision.collectedat)
     return success_response(data)