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-ingest-transcript-to-memory.timer

@@ -4,8 +4,8 @@ Description=OpenClaw Second-Brain ingest transcript -> memory (every 1 min)
 [Timer]
 OnBootSec=30s
 OnUnitActiveSec=60s
+Persistent=true
 Unit=openclaw-secondbrain-ingest-transcript-to-memory.service
 
 [Install]
 WantedBy=timers.target
-