added systemd unit

systemd/signal-light-api.service

@@ -0,0 +1,27 @@
+[Unit]
+Description=Raspberry Pi Signal Light API
+After=network.target
+
+[Service]
+Type=simple
+
+User=pi
+Group=pi
+
+WorkingDirectory=/opt/signal-light-api
+
+Environment="PATH=/opt/signal-light-api/bin"
+
+ExecStart=/opt/signal-light-api/bin/uvicorn main:app \
+    --host 0.0.0.0 \
+    --port 8000
+
+Restart=always
+RestartSec=5
+
+# Send stdout/stderr to the journal
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=multi-user.target