Rename startup script and service files for better clarity

• Renamed start_lightrag.sh to lightrag-api
• Renamed service file to lightrag.service
• Updated service file to use new script name
• Made file names more consistent

lightrag.service.example

@@ -0,0 +1,17 @@
+[Unit]
+Description=LightRAG XYJ Ollama Service
+After=network.target
+
+[Service]
+Type=simple
+User=netman
+# Memory settings
+MemoryHigh=8G
+MemoryMax=12G
+WorkingDirectory=/home/netman/lightrag-xyj
+ExecStart=/home/netman/lightrag-xyj/lightrag-api
+Restart=always
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target