fix: 修复启动导入错误 router → routes

main.py 错误地从 backend.app.api.router 导入 api_router，
实际定义在 backend.app.api.routes 中。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -9,7 +9,7 @@ from fastapi import Request
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 
-from backend.app.api.router import api_router
+from backend.app.api.routes import api_router
 from backend.app.core.config import get_settings
 from backend.app.core.exceptions import AppException