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.

.gitignore

@@ -4,6 +4,7 @@ bin/
 *.out
 
 # Local runtime data
+data/
 backend/static/uploads/*
 !backend/static/uploads/.gitkeep