refactor: 调整项目结构为单向依赖

backend/internal/service/account_group.go

@@ -0,0 +1,13 @@
+package service
+
+import "time"
+
+type AccountGroup struct {
+	AccountID int64
+	GroupID   int64
+	Priority  int
+	CreatedAt time.Time
+
+	Account *Account
+	Group   *Group
+}