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)

fastapi_app.py

@@ -454,7 +454,7 @@ def api_insights(limit: int = Query(8, ge=1, le=50)):
 
 @app.get("/api/graph")
 def api_graph(
-    limit_nodes: int = Query(0, ge=0, le=50000),
+    limit_nodes: int = Query(500, ge=0, le=50000),
     limit_edges: int = Query(0, ge=0, le=200000),
 ):
     """