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/cmd/warpbox/main.go

@@ -30,13 +30,13 @@ func main() {
 
 	server, err := httpserver.New(cfg, logger)
 	if err != nil {
-		logger.Error("failed to create server", "source", "startup", "error", err.Error())
+		logger.Error("failed to create server", "source", "startup", "severity", "error", "error", err.Error())
 		os.Exit(1)
 	}
 
 	errs := make(chan error, 1)
 	go func() {
-		logger.Info("warpbox server starting", "source", "startup", "addr", cfg.Addr, "env", cfg.Environment)
+		logger.Info("warpbox server starting", "source", "startup", "severity", "dev", "addr", cfg.Addr, "env", cfg.Environment)
 		errs <- server.ListenAndServe()
 	}()
 
@@ -46,18 +46,18 @@ func main() {
 	select {
 	case err := <-errs:
 		if err != nil && !errors.Is(err, http.ErrServerClosed) {
-			logger.Error("server stopped unexpectedly", "source", "startup", "error", err.Error())
+			logger.Error("server stopped unexpectedly", "source", "startup", "severity", "error", "error", err.Error())
 			os.Exit(1)
 		}
 	case sig := <-shutdown:
-		logger.Info("shutdown signal received", "source", "startup", "signal", sig.String())
+		logger.Info("shutdown signal received", "source", "startup", "severity", "dev", "signal", sig.String())
 		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 		defer cancel()
 
 		if err := server.Shutdown(ctx); err != nil {
-			logger.Error("graceful shutdown failed", "source", "startup", "error", err.Error())
+			logger.Error("graceful shutdown failed", "source", "startup", "severity", "error", "error", err.Error())
 			os.Exit(1)
 		}
-		logger.Info("server stopped", "source", "startup")
+		logger.Info("server stopped", "source", "startup", "severity", "dev")
 	}
 }