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

frontend/src/router/index.ts

@@ -1,5 +1,6 @@
 import { createRouter, createWebHistory } from 'vue-router'
 import { useAuthStore } from '@/stores/auth'
+import { Terminal } from 'lucide-vue-next'
 
 const router = createRouter({
   history: createWebHistory(),
@@ -69,6 +70,11 @@ const router = createRouter({
           name: 'settings',
           component: () => import('@/views/SettingsView.vue'),
         },
+        {
+          path: 'logs',
+          name: 'logs',
+          component: () => import('@/views/LogView.vue'),
+        },
       ],
     },
   ],