fix(graph): Set default limit_nodes=500 to prevent browser overload

Before: limit_nodes defaulted to 0 (unlimited) causing 51052 nodes to load
After: limit_nodes defaults to 500 for reasonable browser performance

Change: Query(0, ge=0, le=50000) -> Query(500, ge=0, le=50000)

systemd/openclaw-secondbrain-dashboard.service

@@ -1,6 +1,8 @@
 [Unit]
 Description=OpenClaw Second-Brain Dashboard (FastAPI)
-After=network.target
+After=network-online.target
+Wants=network-online.target
+PartOf=openclaw-secondbrain.target
 
 [Service]
 Type=simple
@@ -13,3 +15,4 @@ RestartSec=2
 
 [Install]
 WantedBy=multi-user.target
+WantedBy=openclaw-secondbrain.target