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.

scripts/run/dev.sh

@@ -14,5 +14,9 @@ set -a
 source "${ENV_FILE}"
 set +a
 
+if [[ "${WARPBOX_DATA_DIR:-}" != /* ]]; then
+  export WARPBOX_DATA_DIR="${ROOT_DIR}/${WARPBOX_DATA_DIR:-data}"
+fi
+
 cd "${ROOT_DIR}/backend"
 exec go run ./cmd/warpbox