Add refresh_data script and startup script for offline data setup

- Introduced `scripts/refresh_data.py` to download UNRATE and WTI data from FRED and store it in a SQLite database.
- Added `startup.sh` script to set up a Python virtual environment and install dependencies for offline use.
- Updated `repo-context.txt` to reflect the new scripts and their purposes.

startup.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+python3 -m venv venv
+source venv/bin/activate
+
+pip install --no-index --find-links=wheelhouse -r requirements.txt
+
+echo "✅  Virtual-env ready (offline install)."