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/go.mod

@@ -2,6 +2,9 @@ module warpbox.dev/backend
 
 go 1.26
 
-require go.etcd.io/bbolt v1.4.3
+require (
+	go.etcd.io/bbolt v1.4.3
+	golang.org/x/image v0.41.0
+)
 
 require golang.org/x/sys v0.29.0 // indirect