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/recoverer.go

@@ -12,6 +12,9 @@ func Recoverer(logger *slog.Logger) Middleware {
 			defer func() {
 				if recovered := recover(); recovered != nil {
 					logger.Error("panic recovered",
+						"source", "panic",
+						"severity", "error",
+						"code", 5001,
 						"error", recovered,
 						"stack", string(debug.Stack()),
 						"request_id", RequestIDFromContext(r.Context()),