refactor: members, stats 라우트 폴더 구조로 변경

- members.js → members/index.js
- stats.js → stats/index.js
- 일관된 폴더 구조로 통일

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/routes/index.js

@@ -1,7 +1,7 @@
 import authRoutes from './auth.js';
-import membersRoutes from './members.js';
+import membersRoutes from './members/index.js';
 import albumsRoutes from './albums/index.js';
-import statsRoutes from './stats.js';
+import statsRoutes from './stats/index.js';
 
 /**
  * 라우트 통합

backend/src/routes/members/index.js

@@ -1,4 +1,4 @@
-import { uploadMemberImage } from '../services/image.js';
+import { uploadMemberImage } from '../../services/image.js';
 
 /**
  * 멤버 라우트