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,

backend/app/models/__init__.py

@@ -8,6 +8,7 @@ from app.models.conversation import Conversation, Message
 from app.models.knowledge_graph import KGNode, KGEdge
 from app.models.memory import MemorySummary, UserMemory
 from app.models.todo import DailyTodo, TodoSource
+from app.models.log import Log, LogType, LogLevel
 
 __all__ = [
     "Base",
@@ -28,4 +29,7 @@ __all__ = [
     "UserMemory",
     "DailyTodo",
     "TodoSource",
+    "Log",
+    "LogType",
+    "LogLevel",
 ]