add graph service and "number of known channels" endpoint

backend/src/main/java/de/cotto/lndmanagej/service/GraphService.java

@@ -0,0 +1,23 @@
+package de.cotto.lndmanagej.service;
+
+import de.cotto.lndmanagej.grpc.GrpcGraph;
+import org.springframework.stereotype.Service;
+
+import java.util.Set;
+
+@Service
+public class GraphService {
+    private final GrpcGraph grpcGraph;
+
+    public GraphService(GrpcGraph grpcGraph) {
+        this.grpcGraph = grpcGraph;
+    }
+
+    public int getNumberOfChannels() {
+        return grpcGraph.getChannelEdges().map(Set::size).orElse(0);
+    }
+
+    public void resetCache() {
+        grpcGraph.resetCache();
+    }
+}