Fix Log model registration - import models before init_db

The Log model was not being registered with SQLAlchemy's metadata,
causing the logs table to not be created on startup.

backend/app/main.py

@@ -2,6 +2,7 @@ from contextlib import asynccontextmanager
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from app.database import init_db
+import app.models  # noqa: F401 - 注册所有模型
 from app.routers import (
     auth_router,
     conversation_router,