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/libs/middleware/logger.go

@@ -41,6 +41,7 @@ func Logger(logger *slog.Logger) Middleware {
 
 			logger.Info("http request",
 				"source", "http",
+				"severity", "dev",
 				"code", status,
 				"method", r.Method,
 				"path", r.URL.Path,