feat(admin): 添加系统日志功能

## 新功能

1. **系统日志数据库**
   - 新增 system_logs 表
   - 支持日志级别：debug/info/warn/error
   - 支持日志分类：auth/user/file/share/system/security
   - 记录用户ID、用户名、IP地址、User-Agent

2. **日志记录**
   - 用户注册成功/失败
   - 用户登录成功/失败（密码错误）
   - 系统操作（日志清理等）

3. **管理员API**
   - GET /api/admin/logs - 查询日志（支持分页和筛选）
   - GET /api/admin/logs/stats - 获取日志统计
   - POST /api/admin/logs/cleanup - 清理旧日志

4. **前端界面**
   - 日志列表展示（时间、级别、分类、内容、用户、IP）
   - 筛选功能（级别、分类、关键词搜索）
   - 分页导航
   - 清理旧日志功能

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/database.js

@@ -148,6 +148,32 @@ function initDatabase() {
     console.error('数据库迁移（密码重置Token）失败:', error);
   }
 
+  // 系统日志表
+  db.exec(`
+    CREATE TABLE IF NOT EXISTS system_logs (
+      id INTEGER PRIMARY KEY AUTOINCREMENT,
+      level TEXT NOT NULL DEFAULT 'info',
+      category TEXT NOT NULL,
+      action TEXT NOT NULL,
+      message TEXT NOT NULL,
+      user_id INTEGER,
+      username TEXT,
+      ip_address TEXT,
+      user_agent TEXT,
+      details TEXT,
+      created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+      FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE SET NULL
+    )
+  `);
+
+  // 日志表索引
+  db.exec(`
+    CREATE INDEX IF NOT EXISTS idx_logs_created_at ON system_logs(created_at);
+    CREATE INDEX IF NOT EXISTS idx_logs_category ON system_logs(category);
+    CREATE INDEX IF NOT EXISTS idx_logs_level ON system_logs(level);
+    CREATE INDEX IF NOT EXISTS idx_logs_user_id ON system_logs(user_id);
+  `);
+
   console.log('数据库初始化完成');
 }
 
@@ -610,6 +636,141 @@ function migrateToV2() {
   }
 }
 
+// 系统日志操作
+const SystemLogDB = {
+  // 日志级别常量
+  LEVELS: {
+    DEBUG: 'debug',
+    INFO: 'info',
+    WARN: 'warn',
+    ERROR: 'error'
+  },
+
+  // 日志分类常量
+  CATEGORIES: {
+    AUTH: 'auth',           // 认证相关（登录、登出、注册）
+    USER: 'user',           // 用户管理（创建、修改、删除、封禁）
+    FILE: 'file',           // 文件操作（上传、下载、删除、重命名）
+    SHARE: 'share',         // 分享操作（创建、删除、访问）
+    SYSTEM: 'system',       // 系统操作（设置修改、服务启动）
+    SECURITY: 'security'    // 安全事件（登录失败、暴力破解、异常访问）
+  },
+
+  // 写入日志
+  log({ level = 'info', category, action, message, userId = null, username = null, ipAddress = null, userAgent = null, details = null }) {
+    try {
+      const stmt = db.prepare(`
+        INSERT INTO system_logs (level, category, action, message, user_id, username, ip_address, user_agent, details)
+        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
+      `);
+
+      const detailsStr = details ? (typeof details === 'string' ? details : JSON.stringify(details)) : null;
+
+      stmt.run(level, category, action, message, userId, username, ipAddress, userAgent, detailsStr);
+    } catch (error) {
+      console.error('写入日志失败:', error);
+    }
+  },
+
+  // 查询日志（支持分页和筛选）
+  query({ page = 1, pageSize = 50, level = null, category = null, userId = null, startDate = null, endDate = null, keyword = null }) {
+    let sql = 'SELECT * FROM system_logs WHERE 1=1';
+    let countSql = 'SELECT COUNT(*) as total FROM system_logs WHERE 1=1';
+    const params = [];
+
+    if (level) {
+      sql += ' AND level = ?';
+      countSql += ' AND level = ?';
+      params.push(level);
+    }
+
+    if (category) {
+      sql += ' AND category = ?';
+      countSql += ' AND category = ?';
+      params.push(category);
+    }
+
+    if (userId) {
+      sql += ' AND user_id = ?';
+      countSql += ' AND user_id = ?';
+      params.push(userId);
+    }
+
+    if (startDate) {
+      sql += ' AND created_at >= ?';
+      countSql += ' AND created_at >= ?';
+      params.push(startDate);
+    }
+
+    if (endDate) {
+      sql += ' AND created_at <= ?';
+      countSql += ' AND created_at <= ?';
+      params.push(endDate);
+    }
+
+    if (keyword) {
+      sql += ' AND (message LIKE ? OR username LIKE ? OR action LIKE ?)';
+      countSql += ' AND (message LIKE ? OR username LIKE ? OR action LIKE ?)';
+      const kw = `%${keyword}%`;
+      params.push(kw, kw, kw);
+    }
+
+    // 获取总数
+    const totalResult = db.prepare(countSql).get(...params);
+    const total = totalResult ? totalResult.total : 0;
+
+    // 分页查询
+    sql += ' ORDER BY created_at DESC LIMIT ? OFFSET ?';
+    const offset = (page - 1) * pageSize;
+
+    const logs = db.prepare(sql).all(...params, pageSize, offset);
+
+    return {
+      logs,
+      total,
+      page,
+      pageSize,
+      totalPages: Math.ceil(total / pageSize)
+    };
+  },
+
+  // 获取最近的日志
+  getRecent(limit = 100) {
+    return db.prepare('SELECT * FROM system_logs ORDER BY created_at DESC LIMIT ?').all(limit);
+  },
+
+  // 按分类统计
+  getStatsByCategory() {
+    return db.prepare(`
+      SELECT category, COUNT(*) as count
+      FROM system_logs
+      GROUP BY category
+      ORDER BY count DESC
+    `).all();
+  },
+
+  // 按日期统计（最近7天）
+  getStatsByDate(days = 7) {
+    return db.prepare(`
+      SELECT DATE(created_at) as date, COUNT(*) as count
+      FROM system_logs
+      WHERE created_at >= datetime('now', '-' || ? || ' days')
+      GROUP BY DATE(created_at)
+      ORDER BY date DESC
+    `).all(days);
+  },
+
+  // 清理旧日志（保留指定天数）
+  cleanup(keepDays = 90) {
+    const result = db.prepare(`
+      DELETE FROM system_logs
+      WHERE created_at < datetime('now', '-' || ? || ' days')
+    `).run(keepDays);
+
+    return result.changes;
+  }
+};
+
 // 初始化数据库
 initDatabase();
 createDefaultAdmin();
@@ -622,5 +783,6 @@ module.exports = {
   ShareDB,
   SettingsDB,
   VerificationDB,
-  PasswordResetTokenDB
+  PasswordResetTokenDB,
+  SystemLogDB
 };