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/middleware/logger.go

@@ -40,6 +40,8 @@ func Logger(logger *slog.Logger) Middleware {
 			}
 
 			logger.Info("http request",
+				"source", "http",
+				"code", status,
 				"method", r.Method,
 				"path", r.URL.Path,
 				"status", status,