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/logging/logger.go

@@ -94,12 +94,12 @@ func applyAttr(entry map[string]any, attr slog.Attr) {
 func severity(level slog.Level) string {
 	switch {
 	case level >= slog.LevelError:
-		return "high"
+		return "error"
 	case level >= slog.LevelWarn:
-		return "medium"
+		return "warn"
 	case level <= slog.LevelDebug:
-		return "low"
+		return "dev"
 	default:
-		return "info"
+		return "dev"
 	}
 }