add database backup and restore admin page

- SQLCipher-encrypted backup creation via VACUUM INTO
- Backup history with auto-pruning (keeps last 5)
- Pre-restore automatic backup for safety
- Restore from history or uploaded file
- Stats display with table breakdown
- Download hook for client-side file download
- SECRET_KEY_DB config for encryption at rest

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod_web/router.ex

@@ -170,6 +170,7 @@ defmodule BerrypodWeb.Router do
       live "/providers/:id/edit", Admin.Providers.Form, :edit
       live "/settings", Admin.Settings, :index
       live "/settings/email", Admin.EmailSettings, :index
+      live "/backup", Admin.Backup, :index
       live "/account", Admin.Account, :index
       live "/pages", Admin.Pages.Index, :index
       live "/pages/new", Admin.Pages.CustomForm, :new