feat: add admin console, cleanup, and thumbnail workers

- Implement a token-authenticated admin console at `/admin` with overview metrics and file management.
- Add a background worker to periodically clean up expired boxes based on `WARPBOX_CLEANUP_EVERY`.
- Add a background worker to generate image and video thumbnails based on `WARPBOX_THUMBNAIL_EVERY`.
- Update file storage paths to use `@each@` and `@thumb@` prefixes to separate original files from thumbnails.
- Add severity fields to startup logs and update configuration templates.

backend/templates/layouts/base.html

@@ -12,7 +12,9 @@
   <meta property="og:description" content="{{.Description}}">
   <meta property="og:type" content="website">
   <meta property="og:url" content="{{.BaseURL}}">
+  {{if .ImageURL}}<meta property="og:image" content="{{.ImageURL}}">{{end}}
   <meta name="twitter:card" content="summary_large_image">
+  {{if .ImageURL}}<meta name="twitter:image" content="{{.ImageURL}}">{{end}}
   <link rel="stylesheet" href="/static/css/app.css">
   <script defer src="/static/js/app.js"></script>
 </head>