feat: add configurable data directory and file-based logging

Introduce the `WARPBOX_DATA_DIR` environment variable to define where runtime data is stored. This directory will house uploaded files, the bbolt metadata database, and application logs.

Changes include:
- Added `WARPBOX_DATA_DIR` to configuration, defaulting to `./data`.
- Implemented a custom logging package that writes JSONL logs to the data directory.
- Updated `.gitignore` and `.env.example` to support the new data directory.
- Documented the runtime data structure in `README.md`.
- Updated the frontend upload script to handle form submission and display results.

backend/libs/httpserver/server.go

@@ -17,7 +17,10 @@ func New(cfg config.Config, logger *slog.Logger) (*http.Server, error) {
 		return nil, err
 	}
 
-	uploadService := services.NewUploadService(cfg.MaxUploadSize)
+	uploadService, err := services.NewUploadService(cfg.MaxUploadSize, cfg.DataDir, cfg.BaseURL, logger)
+	if err != nil {
+		return nil, err
+	}
 	app := handlers.NewApp(cfg, logger, renderer, uploadService)
 
 	router := http.NewServeMux()
@@ -32,11 +35,18 @@ func New(cfg config.Config, logger *slog.Logger) (*http.Server, error) {
 		middleware.Logger(logger),
 	)
 
-	return &http.Server{
+	server := &http.Server{
 		Addr:         cfg.Addr,
 		Handler:      handler,
 		ReadTimeout:  cfg.ReadTimeout,
 		WriteTimeout: cfg.WriteTimeout,
 		IdleTimeout:  cfg.IdleTimeout,
-	}, nil
+	}
+	server.RegisterOnShutdown(func() {
+		if err := uploadService.Close(); err != nil {
+			logger.Error("failed to close upload service", "source", "shutdown", "error", err.Error())
+		}
+	})
+
+	return server, nil
 }