fix(upload): render_template_string import 추가

- 모바일 업로드 페이지 500 에러 수정
- Flask import에 render_template_string 추가

backend/app.py

@@ -16,7 +16,7 @@ if sys.platform == 'win32':
     sys.stderr = io.TextIOWrapper(sys.stderr.buffer, encoding='utf-8')
     os.environ.setdefault('PYTHONIOENCODING', 'utf-8')
 
-from flask import Flask, request, render_template, jsonify, redirect, url_for, session
+from flask import Flask, request, render_template, render_template_string, jsonify, redirect, url_for, session
 import hashlib
 import base64
 import secrets