Add log system with three log types (agent/system/chat)

Implemented a complete log system for tracking:
- Agent logs:智能体调用
- System logs: 系统运行
- Chat logs: 问答对话

Backend:
- Log model with type, level, user_id, message, source, duration_ms
- LogService with methods for logging and querying
- API endpoints: GET /api/logs, GET /api/logs/stats, GET /api/logs/recent

Frontend:
- LogView.vue with filters, stats, pagination, auto-refresh
- log.ts API client with TypeScript interfaces
- Added "运行日志" nav item to sidebar

backend/app/routers/__init__.py

@@ -9,3 +9,4 @@ from app.routers.todo import router as todo_router
 from app.routers.settings import router as settings_router
 from app.routers.folder import router as folder_router
 from app.routers.skill import router as skill_router
+from app.routers.log import router as log_router