Add max nodes limit for graph retrieval of networkX

• Set MAX_GRAPH_NODES env var (default 1000)
• Change edge type to "RELATED"

.env.example

@@ -3,6 +3,7 @@
 # PORT=9621
 # WORKERS=1
 # NAMESPACE_PREFIX=lightrag  # separating data from difference Lightrag instances
+# MAX_GRAPH_NODES=1000       # Max nodes return from grap retrieval
 # CORS_ORIGINS=http://localhost:3000,http://localhost:8080
 
 ### Optional SSL Configuration