Footer storage/backend display and a staff role that actually grants staff

Footer:
- Left is now 'Backend Storage:' with a capacity bar (blue, peach at 80%,
  red at 95% per DESIGN.md) and a used/total/free tooltip; the watched
  folder path is no longer printed. /api/status/ returns a compact storage
  summary instead of the path (the full storage page still shows paths to
  authenticated users).
- Centre shows the backend API origin (empty = same origin). Staff get a
  link to /setup to point the browser elsewhere; everyone else sees it as
  plain text. The Account 'Backend connection' card is gone — this is
  installation plumbing, not a per-user setting.
- Design spec updated to match.

Staff role:
- The custom role did nothing on several endpoints that only accepted
  Django's is_staff/is_superuser. One canonical check now exists:
  User.is_app_staff (superuser, Django staff, or the staff role), used by
  the stats/users APIs, item object permissions, can_delete, upload/
  similarity/download/match querysets, and the management commands
  (which also pick staff-role accounts for e621 sync/match and file
  ownership).

Verified with a role-only staff account (is_staff/is_superuser false):
stats/users 200, all 32 downloads + 2 scans visible, others' items
editable; the same account as role=user gets 403 for all of those.

backend/apps/library/views.py

@@ -242,11 +242,7 @@ class MediaItemViewSet(
 
     def _can_match(self, request, item):
         user = request.user
-        return bool(
-            user.is_superuser
-            or user.role == user.ROLE_STAFF
-            or item.uploaded_by_id == user.id
-        )
+        return bool(user.is_app_staff or item.uploaded_by_id == user.id)
 
     @action(detail=True, methods=["post"], permission_classes=[CanUpload])
     def match(self, request, pk=None):
@@ -424,7 +420,7 @@ class DownloadTaskViewSet(
         if post_id.isdigit():
             queryset = queryset.filter(post_id=int(post_id))
         user = self.request.user
-        if not (user.is_staff or user.is_superuser):
+        if not user.is_app_staff:
             queryset = queryset.filter(user=user)
         return queryset
 
@@ -491,7 +487,7 @@ class MatchTaskViewSet(
         reap_stale_match_tasks()
         queryset = MatchTask.objects.all()
         user = self.request.user
-        if not (user.is_staff or user.is_superuser):
+        if not user.is_app_staff:
             queryset = queryset.filter(user=user)
         return queryset