Initial commit: Busbar Designer

Web tool for designing nickel/copper busbars over cylindrical-cell battery
packs (21700, 18650) in hex holders. Flask + build123d backend exports
STEP/DXF/SVG; vanilla JS frontend with live preview, multi-project SQLite
persistence, snapshot history.

Deploy scripts in deploy/ (proxmox-lxc.sh, install.sh, update.sh).

deploy/busbar-designer.service

@@ -0,0 +1,38 @@
+; systemd unit for running busbar-designer in a Proxmox LXC (or any Linux VM)
+; without Docker. Assumes the project lives at /opt/busbar-designer and you've
+; created a venv there with `python3 -m venv .venv && .venv/bin/pip install -r
+; requirements.txt gunicorn`.
+;
+; Install:
+;   sudo cp deploy/busbar-designer.service /etc/systemd/system/
+;   sudo systemctl daemon-reload
+;   sudo systemctl enable --now busbar-designer
+;
+; Logs:  journalctl -u busbar-designer -f
+[Unit]
+Description=Busbar Designer (Flask + build123d)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=busbar
+Group=busbar
+WorkingDirectory=/opt/busbar-designer
+Environment=HOST=0.0.0.0
+Environment=PORT=5000
+Environment=FLASK_DEBUG=0
+Environment=PATH=/opt/busbar-designer/.venv/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+ExecStart=/opt/busbar-designer/.venv/bin/gunicorn --bind=0.0.0.0:5000 --workers=2 --threads=2 --timeout=120 app:app
+Restart=on-failure
+RestartSec=5
+
+; Hardening
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectSystem=strict
+ProtectHome=true
+ReadWritePaths=/opt/busbar-designer
+
+[Install]
+WantedBy=multi-user.target