chore: add systemd service file for backend deployment

backend/vm-bench-backend.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=VM Bench Backend — Proxmox Image Conversion Engine
+After=network.target
+
+[Service]
+Type=simple
+User=root
+WorkingDirectory=/mnt/converter/backend
+Environment=PATH=/usr/local/bin:/usr/bin:/bin
+ExecStart=/usr/bin/python3 -m uvicorn app:app --host 0.0.0.0 --port 9000
+Restart=always
+RestartSec=3
+
+[Install]
+WantedBy=multi-user.target