Feat: 实现知识图谱数据库的检索方法，并对外提供接口

backend/services/service.go

@@ -0,0 +1,11 @@
+package services
+
+import "knowledge-graph-backend/models"
+
+type GraphService interface {
+	GetGraphData() models.GraphData
+	GetNodeByID(id string) (models.Node, bool)
+	SearchNodes(query string) []models.Node
+	GetNeighbors(nodeID string) (models.NeighborResponse, bool)
+	GetStats() map[string]int
+}