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/management/commands/scan_files.py

@@ -4,6 +4,7 @@ from pathlib import Path
 from django.conf import settings
 from django.contrib.auth import get_user_model
 from django.core.management.base import BaseCommand
+from django.db.models import Q
 
 from apps.library.models import MediaLocation
 from apps.library.services import ALLOWED_EXTENSIONS, index_file
@@ -42,11 +43,19 @@ class Command(BaseCommand):
                 )
                 return
         else:
-            owner = User.objects.filter(is_superuser=True).order_by("id").first()
+            owner = (
+                User.objects.filter(
+                    Q(is_superuser=True)
+                    | Q(is_staff=True)
+                    | Q(role=User.ROLE_STAFF)
+                )
+                .order_by("-is_superuser", "id")
+                .first()
+            )
             if owner is None:
                 self.stderr.write(
                     self.style.WARNING(
-                        "No superuser found; scanned files will have no owner. "
+                        "No staff user found; scanned files will have no owner. "
                         "Use --user <name> to assign one."
                     )
                 )