feat(backup): add database backup/restore with scheduled retention and admin dashboard

- Add BackupRecord model for tracking backup archives
- Add migration 021_add_backup_records
- Add backup configuration settings (backup_enabled, backup_dir, backup_remote_destination, backup_remote_folder, backup_retain_hourly/daily/weekly)
- Add backup_tasks.py with create_backup, cleanup_old_backups, and helpers
- Register hourly/daily/weekly Celery beat schedules
- Add /api/admin/backup/* REST endpoints (list, create, download, restore, delete, cleanup)
- Add /admin/backup dashboard view and template
- Add backup link to admin dropdown navigation in base.html
- Add backup settings to SETTING_METADATA in settings_service.py
- Add comprehensive test suite (39 tests passing)

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/base.html

@@ -160,6 +160,9 @@
                   <a href="/admin/queue" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                     <i class="fas fa-stream w-4 mr-2 text-blue-500" aria-hidden="true"></i> Queue Monitor
                   </a>
+                  <a href="/admin/backup" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
+                    <i class="fas fa-database w-4 mr-2 text-green-600" aria-hidden="true"></i> Backup &amp; Restore
+                  </a>
                 </div>
               </div>
             </div>
@@ -298,6 +301,9 @@
                   <a href="/admin/queue" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
                     <i class="fas fa-stream mr-2 text-blue-400" aria-hidden="true"></i> Queue Monitor
                   </a>
+                  <a href="/admin/backup" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
+                    <i class="fas fa-database mr-2 text-green-500" aria-hidden="true"></i> Backup &amp; Restore
+                  </a>
                 </div>
               </div>