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/api/log.ts

@@ -0,0 +1,61 @@
+import api from './index'
+import type { AxiosResponse } from 'axios'
+
+export interface Log {
+  id: string
+  level: 'debug' | 'info' | 'warning' | 'error'
+  type: 'agent' | 'system' | 'chat'
+  user_id: string | null
+  message: string
+  source: string | null
+  details: string | null
+  duration_ms: string | null
+  created_at: string
+  updated_at: string
+}
+
+export interface LogStats {
+  total: number
+  by_type: {
+    agent: number
+    system: number
+    chat: number
+  }
+  by_level: {
+    debug: number
+    info: number
+    warning: number
+    error: number
+  }
+}
+
+export interface LogQueryResult {
+  logs: Log[]
+  total: number
+  page: number
+  page_size: number
+}
+
+export const logApi = {
+  list: (params?: {
+    log_type?: string
+    level?: string
+    source?: string
+    page?: number
+    page_size?: number
+  }): Promise<AxiosResponse<LogQueryResult>> => {
+    return api.get('/api/logs', { params })
+  },
+
+  getStats: (hours?: number): Promise<AxiosResponse<LogStats>> => {
+    return api.get('/api/logs/stats', { params: { hours } })
+  },
+
+  getRecent: (params?: {
+    log_type?: string
+    hours?: number
+    limit?: number
+  }): Promise<AxiosResponse<Log[]>> => {
+    return api.get('/api/logs/recent', { params })
+  },
+}